diff --git a/NEWS.md b/NEWS.md
index 7b2edbbdc..8561e01e2 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -146,6 +146,19 @@ stability of steady states.
   that does not hold a density is now an error; it used to be swallowed silently
   by `...`.
 
+- The resource can now be shown on length-based plots. `resource_params()`
+  gains the weight-length parameters `a` and `b`, defaulting to the equivalent
+  spherical diameter of an organism with the density of water, `a = pi/6` and
+  `b = 3`, which is the convention plankton ecology uses for a composite of many
+  taxa. `plotSpectra(params, size_axis = "l")` therefore includes the resource
+  spectrum, where it used to drop it silently, and the resource array plots and
+  `animate()` gain `size_axis` and `llim`. The parameters feed none of the
+  rates. Note that the resource then sits on the length axis at its own
+  convention: a fish of a given weight is about 3.7 times longer than a sphere
+  of that weight. That difference is real rather than an artefact — a 1 mg
+  copepod really is shorter than a 1 mg fish larva — but it does mean the
+  resource and the species are measured differently.
+
 - `plotYieldObservedVsModel()` gains a `gear` argument that restricts the
   comparison to the catch of the selected gears. Both the model yield and the
   observed yield are then taken from those gears only, so in a model where
diff --git a/R/ArrayResourceBySize-class.R b/R/ArrayResourceBySize-class.R
index df918df59..3b548ab3b 100644
--- a/R/ArrayResourceBySize-class.R
+++ b/R/ArrayResourceBySize-class.R
@@ -149,9 +149,14 @@ print.summary.ArrayResourceBySize <- function(x, ...) {
 #' @param wlim A numeric vector of length two providing lower and upper
 #'   limits for the weight (x) axis. Use `NA` to refer to the existing
 #'   minimum or maximum.
+#' @param llim A numeric vector of length two providing lower and upper limits
+#'   for the length (x) axis when `size_axis = "l"`. Use `NA` to refer to the
+#'   existing minimum or maximum.
 #' @param ylim A numeric vector of length two providing lower and upper
 #'   limits for the value (y) axis. Use `NA` to refer to the existing
 #'   minimum or maximum.
+#' @param size_axis Whether to plot size as weight (`"w"`, default) or length
+#'   (`"l"`), using the weight-length relationship in [resource_params()].
 #' @param per_log_size For an array that holds a density, whether to plot it
 #'   per logarithmic size (`TRUE`) rather than per size (`FALSE`). The default,
 #'   `NULL`, plots the density as it stands. An error for an array that does not
@@ -170,9 +175,12 @@ print.summary.ArrayResourceBySize <- function(x, ...) {
 #' }
 plot.ArrayResourceBySize <- function(x, return_data = FALSE,
                                      log_x = TRUE, log_y = TRUE, log = NULL,
-                                     wlim = c(NA, NA), ylim = c(NA, NA),
+                                     wlim = c(NA, NA), llim = c(NA, NA),
+                                     ylim = c(NA, NA),
+                                     size_axis = c("w", "l"),
                                      per_log_size = NULL,
                                      y_ticks = 6, ...) {
+    size_axis <- plot_size_axis(size_axis)
     check_per_log_size(x, per_log_size)
     log_y <- array_log_y(x, log_y, log, !missing(log_y))
     log_axes <- parsePlotLog(log, log_x = log_x, log_y = log_y)
@@ -180,27 +188,29 @@ plot.ArrayResourceBySize <- function(x, return_data = FALSE,
     log_y <- log_axes$log_y
 
     assert_that(length(wlim) == 2,
+                length(llim) == 2,
                 length(ylim) == 2)
     params <- attr(x, "params")
 
     plot_dat <- prepare_ArrayResourceBySize_plot_data(x, wlim = wlim)
-    # The resource has no weight-length relationship, so it has no length axis.
-    # Expressing a density per logarithmic weight needs no such relationship,
-    # though, so that much is available here.
-    plot_dat <- convert_plot_density_axis(plot_dat, params, "w",
+    plot_dat <- convert_plot_density_axis(plot_dat, params, size_axis,
                                           density_wrt = array_density_wrt(x),
                                           per_log_size = per_log_size)
+    if (identical(size_axis, "l")) {
+        plot_dat <- filter_plot_length_limits(plot_dat, llim)
+    }
 
     if (return_data) return(plot_dat)
 
-    y_label <- array_y_label(x, default = "value", per_log_size = per_log_size)
+    y_label <- array_y_label(x, default = "value", size_axis = size_axis,
+                             per_log_size = per_log_size)
 
     ylim <- array_ylim(x, ylim, log_y, plot_dat[[2]])
-    plotDataFrame(plot_dat, params, xlab = "Weight (g)",
+    plotDataFrame(plot_dat, params, xlab = plot_size_xlab(size_axis),
                   ylab = y_label,
                   xtrans = if (log_x) "log10" else "identity",
                   ytrans = if (log_y) "log10" else "identity",
-                  xlim = wlim, ylim = ylim,
+                  xlim = plot_size_xlim(wlim, size_axis, llim), ylim = ylim,
                   y_ticks = y_ticks, legend_var = "Legend")
 }
 
@@ -721,13 +731,7 @@ animate.ArrayTimeByResourceBySize <- function(x, species = NULL,
                 length(wlim) == 2, length(ylim) == 2, length(tlim) == 2)
     warn_unused_resource_args(species, total, background)
     check_per_log_size(x, per_log_size)
-    # The length axis is derived from each species' weight-length parameters,
-    # and the resource is not a species.
     size_axis <- plot_size_axis(size_axis)
-    if (identical(size_axis, "l")) {
-        stop("A length axis is not available for resource arrays, because the ",
-             "weight-length relationship is a species parameter.")
-    }
     log_axes <- parsePlotLog(log, log_x = log_x, log_y = log_y)
     log_x <- log_axes$log_x
     log_y <- log_axes$log_y
diff --git a/R/ArraySpeciesBySize-class.R b/R/ArraySpeciesBySize-class.R
index c265580f5..65c3f9065 100644
--- a/R/ArraySpeciesBySize-class.R
+++ b/R/ArraySpeciesBySize-class.R
@@ -473,9 +473,8 @@ parsePlotLog <- function(log, log_x = FALSE, log_y = FALSE) {
 #'       limits for the length (x) axis when `size_axis = "l"`. Use `NA` to
 #'       refer to the existing minimum or maximum.}
 #'     \item{`size_axis`}{Whether to plot size as weight (`"w"`, default) or
-#'       length (`"l"`), using the allometric weight-length relationship. Not
-#'       available for the resource classes, because the weight-length
-#'       relationship is a species parameter.}
+#'       length (`"l"`), using the allometric weight-length relationship of
+#'       each species, or of the resource, see [resource_params()].}
 #'     \item{`per_log_size`}{For an array that holds a density, whether to plot
 #'       it per logarithmic size (`TRUE`) rather than per size (`FALSE`). The
 #'       default, `NULL`, plots the density as it stands. Unlike `size_axis`
@@ -605,9 +604,8 @@ plot2.ArraySpeciesBySize <- function(x, y, name1 = "First", name2 = "Second",
 #'       limits for the length (x) axis when `size_axis = "l"`. Use `NA` to
 #'       refer to the existing minimum or maximum.}
 #'     \item{`size_axis`}{Whether to plot size as weight (`"w"`, default) or
-#'       length (`"l"`), using the allometric weight-length relationship. Not
-#'       available for the resource classes, because the weight-length
-#'       relationship is a species parameter.}
+#'       length (`"l"`), using the allometric weight-length relationship of
+#'       each species, or of the resource, see [resource_params()].}
 #'   }
 #'
 #'   **For `ArrayTimeBySpecies` methods:**
@@ -895,9 +893,8 @@ check_per_log_size <- function(x, per_log_size) {
 #'       limits for the length (x) axis when `size_axis = "l"`. Use `NA` to
 #'       refer to the existing minimum or maximum.}
 #'     \item{`size_axis`}{Whether to plot size as weight (`"w"`, default) or
-#'       length (`"l"`), using the allometric weight-length relationship. Not
-#'       available for the resource classes, because the weight-length
-#'       relationship is a species parameter.}
+#'       length (`"l"`), using the allometric weight-length relationship of
+#'       each species, or of the resource, see [resource_params()].}
 #'     \item{`per_log_size`}{For an array that holds a density, whether to plot
 #'       it per logarithmic size (`TRUE`) rather than per size (`FALSE`). The
 #'       default, `NULL`, plots the density as it stands. Unlike `size_axis`
diff --git a/R/animateSpectra.R b/R/animateSpectra.R
index c3250fa0b..8b0517f66 100644
--- a/R/animateSpectra.R
+++ b/R/animateSpectra.R
@@ -24,8 +24,7 @@
 #' * **`ArrayTimeByResourceBySize`** — animates the size-resolved resource
 #'   quantity returned by [NResource()] on a `MizerSim` object. There is only a
 #'   single resource spectrum, so `species`, `total` and `background` do nothing
-#'   and warn if set, and `size_axis = "l"` is an error because the
-#'   weight-length relationship is a species parameter.
+#'   and warn if set.
 #'
 #' Species linecolours and linetypes follow `params@linecolour` and
 #' `params@linetype`.
@@ -52,9 +51,9 @@
 #'   it too. An error for an array that does not hold a density. The `MizerSim`
 #'   method has its own `per_log_size`, described below.
 #' @param size_axis Whether to plot size as weight (`"w"`, default) or length
-#'   (`"l"`), using the allometric weight-length relationship. Number and
-#'   biomass densities are transformed to match the chosen axis. The
-#'   `ArrayTimeByResourceBySize` method supports only `"w"`.
+#'   (`"l"`), using the allometric weight-length relationship of each species,
+#'   or of the resource, see [resource_params()]. Number and biomass densities
+#'   are transformed to match the chosen axis.
 #' @param total A boolean value that determines whether the total over all
 #'   selected species is plotted as an additional trace called `"Total"`.
 #'   Default is `FALSE`. Not used by the `ArrayTimeByResourceBySize` method,
diff --git a/R/plots.R b/R/plots.R
index d2b80013e..43ad512e7 100644
--- a/R/plots.R
+++ b/R/plots.R
@@ -726,10 +726,43 @@ array_log_y <- function(x, log_y, log, given) {
     log_y
 }
 
+#' The weight-length parameters to plot each row of plotting data with
+#'
+#' A length axis needs an allometric weight-length relationship for every line
+#' on the plot. The species take theirs from their species parameters and the
+#' resource takes its from [resource_params()], where it defaults to the
+#' equivalent spherical diameter (see [resource_length_params()]). Anything else
+#' — the "Total" row, for instance — has none, and is reported as `NA` so that
+#' the caller can leave it out.
+#'
+#' @param species A vector of the species names in the plotting data.
+#' @param params A MizerParams object providing the weight-length parameters.
+#' @return A data frame with columns `a` and `b`, one row for each element of
+#'   `species`, holding `NA` where no relationship is known.
+#' @keywords internal
+plot_length_params <- function(species, params) {
+    sp <- params@species_params
+    if (!all(c("a", "b") %in% names(sp))) {
+        stop("The species parameter data frame must contain columns ",
+             "'a' and 'b'.")
+    }
+    species <- as.character(species)
+    idx <- match(species, as.character(sp$species))
+    out <- data.frame(a = sp[["a"]][idx], b = sp[["b"]][idx])
+    is_resource <- species == "Resource"
+    if (any(is_resource)) {
+        rp <- resource_length_params(params)
+        out$a[is_resource] <- rp$a
+        out$b[is_resource] <- rp$b
+    }
+    out
+}
+
 #' Convert plotting data from weight to length
 #'
 #' When `size_axis = "l"`, adds a length column `l` computed from the weight
-#' column `w` using each species' weight-length relationship. For
+#' column `w` using the weight-length relationship of each line, see
+#' [plot_length_params()]. Rows with no such relationship are dropped. For
 #' `size_axis = "w"` the data is returned unchanged.
 #'
 #' @param plot_dat A data frame of plotting data with a `w` column and a species
@@ -761,10 +794,10 @@ convert_plot_size_axis <- function(plot_dat, params, size_axis,
              "` column.")
     }
 
-    species <- as.character(plot_dat[[species_col]])
-    species_idx <- match(species, as.character(params@species_params$species))
-    plot_dat <- plot_dat[!is.na(species_idx), , drop = FALSE]
-    species_idx <- species_idx[!is.na(species_idx)]
+    ab <- plot_length_params(plot_dat[[species_col]], params)
+    known <- !is.na(ab$a) & !is.na(ab$b)
+    plot_dat <- plot_dat[known, , drop = FALSE]
+    ab <- ab[known, , drop = FALSE]
     if (nrow(plot_dat) == 0) {
         if (!drop_w) {
             plot_dat$l <- numeric(0)
@@ -774,8 +807,7 @@ convert_plot_size_axis <- function(plot_dat, params, size_axis,
         }
         return(plot_dat[, setdiff(names(plot_dat), "w"), drop = FALSE])
     }
-    sp <- params@species_params[species_idx, , drop = FALSE]
-    plot_dat$l <- w2l(plot_dat$w, sp)
+    plot_dat$l <- (plot_dat$w / ab$a)^(1 / ab$b)
     if (drop_w) {
         return(plot_dat[, c("l", setdiff(names(plot_dat), c("l", "w"))),
                         drop = FALSE])
@@ -983,10 +1015,8 @@ convert_plot_density_axis <- function(plot_dat, params, size_axis,
     }
     if (needs_jacobian && nrow(plot_dat) > 0) {
         if (needs_length) {
-            species_idx <- match(as.character(plot_dat[[species_col]]),
-                                 as.character(params@species_params$species))
             l <- plot_dat$l
-            b <- params@species_params$b[species_idx]
+            b <- plot_length_params(plot_dat[[species_col]], params)$b
         } else {
             # Unused by a Jacobian between two weight-based measures. Passing
             # NA rather than a placeholder keeps a mistake here loud.
diff --git a/R/resource_dynamics.R b/R/resource_dynamics.R
index 000996839..968ceeeb5 100644
--- a/R/resource_dynamics.R
+++ b/R/resource_dynamics.R
@@ -59,6 +59,17 @@ resource_constant <- function(params, n_pp, ...) {
 #' Unlike the carrying capacity, however, the initial resource abundance is
 #' **not** updated when you subsequently change `kappa` (or call [setResource()]).
 #'
+#' The resource parameters `a` and `b` give the allometric weight-length
+#' relationship \eqn{w = a l^b} of the resource, with \eqn{w} in grams and
+#' \eqn{l} in centimetres. They feed none of the rates; they exist so that the
+#' resource can be shown on the length-based plots (`size_axis = "l"`) alongside
+#' the species. They default to the equivalent spherical diameter of an organism
+#' with the density of water, \eqn{a = \pi/6} and \eqn{b = 3}, which is the
+#' convention plankton ecology uses for a composite of many taxa. This is a
+#' different convention from the one the species use, so the resource and the
+#' species each sit on the length axis at their own; see
+#' [resource_length_defaults].
+#'
 #' Assigning to `resource_params` only rebuilds the size-dependent resource rate
 #' and capacity arrays from these scalars (leaving any arrays you have set
 #' manually untouched). Changing `lambda` also recalculates any `q` and `gamma`
@@ -97,6 +108,12 @@ resource_params <- function(params) {
     if (!is.null(value$r_pp)) {
         assert_that(is.number(value$r_pp), value$r_pp >= 0)
     }
+    if (!is.null(value[["a"]])) {
+        assert_that(is.number(value[["a"]]), value[["a"]] > 0)
+    }
+    if (!is.null(value[["b"]])) {
+        assert_that(is.number(value[["b"]]), value[["b"]] > 0)
+    }
     
     scalars <- c("kappa", "lambda", "n", "w_pp_cutoff", "r_pp")
     changed <- scalars[vapply(scalars, function(scalar) {
diff --git a/R/setResource.R b/R/setResource.R
index 01301effd..3d2e02be7 100644
--- a/R/setResource.R
+++ b/R/setResource.R
@@ -207,6 +207,13 @@ setResource.MizerParams <- function(params,
     params@resource_params[["lambda"]] <- lambda
     params@resource_params[["n"]] <- n
     params@resource_params[["w_pp_cutoff"]] <- w_pp_cutoff
+    # The weight-length parameters feed no rate, so they are not arguments
+    # here; they are filled in so that `resource_params()` shows them and the
+    # user can change them there.
+    params@resource_params[["a"]] <-
+        params@resource_params[["a"]] %||% resource_length_defaults$a
+    params@resource_params[["b"]] <-
+        params@resource_params[["b"]] %||% resource_length_defaults$b
 
     if (!is.null(resource_capacity) && !is.null(resource_level)) {
         stop("You should specify only either 'resource_level' or 'resource_capacity'.")
@@ -557,6 +564,45 @@ resource_dynamics <- function(params) {
     setResource(params, resource_dynamics = value, balance = balance)
 }
 
+#' Default weight-length parameters for the resource
+#'
+#' The resource is a composite of everything from bacteria to
+#' macrozooplankton, so it has no taxonomic length-weight relationship. The
+#' default is the geometric one that plankton ecology uses instead: the
+#' **equivalent spherical diameter** of an organism with the density of water,
+#' \deqn{w = \frac{\pi}{6} l^3,}
+#' with \eqn{w} in grams and \eqn{l} in centimetres. On a mizer size grid this
+#' puts the smallest resource sizes at a fraction of a micrometre and a
+#' milligram organism at about a millimetre, which is the right order for
+#' bacteria and copepods respectively.
+#'
+#' Note that this is a different convention from the one the species use: a fish
+#' of a given weight is longer than a sphere of the same weight, by a factor
+#' \eqn{(a_{fish}/a_{resource})^{-1/3}}, about 3.7 for the mizer default
+#' `a = 0.01`. That difference is real rather than an artefact — a 1 mg copepod
+#' really is shorter than a 1 mg fish larva — but it does mean the resource and
+#' the species sit on the plot at their own conventions.
+#'
+#' @format A list with entries `a` and `b`.
+#' @seealso [resource_params()]
+#' @keywords internal
+resource_length_defaults <- list(a = pi / 6, b = 3)
+
+#' The weight-length parameters of the resource
+#'
+#' Reads `a` and `b` from [resource_params()], falling back to
+#' [resource_length_defaults] for a model that does not set them — which is
+#' every model built before these parameters existed.
+#'
+#' @param params A MizerParams object.
+#' @return A list with entries `a` and `b`.
+#' @keywords internal
+resource_length_params <- function(params) {
+    rp <- params@resource_params
+    list(a = rp[["a"]] %||% resource_length_defaults$a,
+         b = rp[["b"]] %||% resource_length_defaults$b)
+}
+
 # The resource capacity that the scalar resource parameters imply, or NULL if
 # they do not determine it. Used both to recompute a capacity that mizer
 # controls and to see whether a frozen one has fallen out of step with the
diff --git a/inst/skills/analyse-and-plot/SKILL.md b/inst/skills/analyse-and-plot/SKILL.md
index 359ddfefb..360273e4f 100644
--- a/inst/skills/analyse-and-plot/SKILL.md
+++ b/inst/skills/analyse-and-plot/SKILL.md
@@ -354,9 +354,13 @@ plotSpectra(sim, species = c("Cod", "Herring"), resource = FALSE)
 plotSpectra(sim, biomass = TRUE, size_axis = "l")     # biomass density against length
 ```
 
-**The resource is dropped from a length axis.** The resource spectrum has no
-  length-weight relationship, so `plotSpectra(params, size_axis = "l")` shows the
-  species only. Use `size_axis = "w"` to see the resource.
+**The resource has its own length convention.** It is a composite of many taxa,
+so instead of a taxonomic weight-length relationship it uses the equivalent
+spherical diameter of an organism with the density of water (`a = pi/6`,
+`b = 3`, in `resource_params()`). It therefore appears on a length axis, but
+measured differently from the fish: a fish of a given weight is about 3.7 times
+longer than a sphere of that weight. That gap at the resource-consumer boundary
+is real biology, not an artefact.
 
 ### Which density a spectrum plot shows
 
diff --git a/man/addPlot.Rd b/man/addPlot.Rd
index deb3aeec0..73eb9359a 100644
--- a/man/addPlot.Rd
+++ b/man/addPlot.Rd
@@ -75,9 +75,8 @@ range (\code{w_min} to \code{w_max}) are removed.}
 limits for the length (x) axis when \code{size_axis = "l"}. Use \code{NA} to
 refer to the existing minimum or maximum.}
 \item{\code{size_axis}}{Whether to plot size as weight (\code{"w"}, default) or
-length (\code{"l"}), using the allometric weight-length relationship. Not
-available for the resource classes, because the weight-length
-relationship is a species parameter.}
+length (\code{"l"}), using the allometric weight-length relationship of
+each species, or of the resource, see \code{\link[=resource_params]{resource_params()}}.}
 \item{\code{per_log_size}}{For an array that holds a density, whether to plot
 it per logarithmic size (\code{TRUE}) rather than per size (\code{FALSE}). The
 default, \code{NULL}, plots the density as it stands. Unlike \code{size_axis}
diff --git a/man/animate.Rd b/man/animate.Rd
index 243211437..e67bd209d 100644
--- a/man/animate.Rd
+++ b/man/animate.Rd
@@ -65,9 +65,9 @@ for the animated time window, e.g. \code{c(1997, 2007)}. Use \code{NA} to apply
 limit at that end. Default is \code{c(NA, NA)}.}
 
 \item{size_axis}{Whether to plot size as weight (\code{"w"}, default) or length
-(\code{"l"}), using the allometric weight-length relationship. Number and
-biomass densities are transformed to match the chosen axis. The
-\code{ArrayTimeByResourceBySize} method supports only \code{"w"}.}
+(\code{"l"}), using the allometric weight-length relationship of each species,
+or of the resource, see \code{\link[=resource_params]{resource_params()}}. Number and biomass densities
+are transformed to match the chosen axis.}
 
 \item{per_log_size}{For an array that holds a density, whether to animate it
 per logarithmic size (\code{TRUE}) rather than per size (\code{FALSE}). The default,
@@ -147,8 +147,7 @@ and \code{total} arguments.
 \item \strong{\code{ArrayTimeByResourceBySize}} — animates the size-resolved resource
 quantity returned by \code{\link[=NResource]{NResource()}} on a \code{MizerSim} object. There is only a
 single resource spectrum, so \code{species}, \code{total} and \code{background} do nothing
-and warn if set, and \code{size_axis = "l"} is an error because the
-weight-length relationship is a species parameter.
+and warn if set.
 }
 
 Species linecolours and linetypes follow \code{params@linecolour} and
diff --git a/man/convert_plot_size_axis.Rd b/man/convert_plot_size_axis.Rd
index 79376f4ad..5adf0a444 100644
--- a/man/convert_plot_size_axis.Rd
+++ b/man/convert_plot_size_axis.Rd
@@ -32,7 +32,8 @@ dropped) when \code{size_axis = "l"}.
 }
 \description{
 When \code{size_axis = "l"}, adds a length column \code{l} computed from the weight
-column \code{w} using each species' weight-length relationship. For
+column \code{w} using the weight-length relationship of each line, see
+\code{\link[=plot_length_params]{plot_length_params()}}. Rows with no such relationship are dropped. For
 \code{size_axis = "w"} the data is returned unchanged.
 }
 \keyword{internal}
diff --git a/man/plot.ArrayResourceBySize.Rd b/man/plot.ArrayResourceBySize.Rd
index 84b7f6bfc..c36a33f65 100644
--- a/man/plot.ArrayResourceBySize.Rd
+++ b/man/plot.ArrayResourceBySize.Rd
@@ -11,7 +11,9 @@
   log_y = TRUE,
   log = NULL,
   wlim = c(NA, NA),
+  llim = c(NA, NA),
   ylim = c(NA, NA),
+  size_axis = c("w", "l"),
   per_log_size = NULL,
   y_ticks = 6,
   ...
@@ -36,10 +38,17 @@ scales, in the same form as the base \code{\link[=plot]{plot()}} argument. For e
 limits for the weight (x) axis. Use \code{NA} to refer to the existing
 minimum or maximum.}
 
+\item{llim}{A numeric vector of length two providing lower and upper limits
+for the length (x) axis when \code{size_axis = "l"}. Use \code{NA} to refer to the
+existing minimum or maximum.}
+
 \item{ylim}{A numeric vector of length two providing lower and upper
 limits for the value (y) axis. Use \code{NA} to refer to the existing
 minimum or maximum.}
 
+\item{size_axis}{Whether to plot size as weight (\code{"w"}, default) or length
+(\code{"l"}), using the weight-length relationship in \code{\link[=resource_params]{resource_params()}}.}
+
 \item{per_log_size}{For an array that holds a density, whether to plot it
 per logarithmic size (\code{TRUE}) rather than per size (\code{FALSE}). The default,
 \code{NULL}, plots the density as it stands. An error for an array that does not
diff --git a/man/plot2.Rd b/man/plot2.Rd
index 932ef589f..f9294f0a9 100644
--- a/man/plot2.Rd
+++ b/man/plot2.Rd
@@ -85,9 +85,8 @@ range (\code{w_min} to \code{w_max}) are removed.}
 limits for the length (x) axis when \code{size_axis = "l"}. Use \code{NA} to
 refer to the existing minimum or maximum.}
 \item{\code{size_axis}}{Whether to plot size as weight (\code{"w"}, default) or
-length (\code{"l"}), using the allometric weight-length relationship. Not
-available for the resource classes, because the weight-length
-relationship is a species parameter.}
+length (\code{"l"}), using the allometric weight-length relationship of
+each species, or of the resource, see \code{\link[=resource_params]{resource_params()}}.}
 \item{\code{per_log_size}}{For an array that holds a density, whether to plot
 it per logarithmic size (\code{TRUE}) rather than per size (\code{FALSE}). The
 default, \code{NULL}, plots the density as it stands. Unlike \code{size_axis}
diff --git a/man/plotRelative.Rd b/man/plotRelative.Rd
index 578570185..02d87112c 100644
--- a/man/plotRelative.Rd
+++ b/man/plotRelative.Rd
@@ -67,9 +67,8 @@ range (\code{w_min} to \code{w_max}) are removed.}
 limits for the length (x) axis when \code{size_axis = "l"}. Use \code{NA} to
 refer to the existing minimum or maximum.}
 \item{\code{size_axis}}{Whether to plot size as weight (\code{"w"}, default) or
-length (\code{"l"}), using the allometric weight-length relationship. Not
-available for the resource classes, because the weight-length
-relationship is a species parameter.}
+length (\code{"l"}), using the allometric weight-length relationship of
+each species, or of the resource, see \code{\link[=resource_params]{resource_params()}}.}
 }
 
 \strong{For \code{ArrayTimeBySpecies} methods:}
diff --git a/man/plot_length_params.Rd b/man/plot_length_params.Rd
new file mode 100644
index 000000000..861f537db
--- /dev/null
+++ b/man/plot_length_params.Rd
@@ -0,0 +1,26 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/plots.R
+\name{plot_length_params}
+\alias{plot_length_params}
+\title{The weight-length parameters to plot each row of plotting data with}
+\usage{
+plot_length_params(species, params)
+}
+\arguments{
+\item{species}{A vector of the species names in the plotting data.}
+
+\item{params}{A MizerParams object providing the weight-length parameters.}
+}
+\value{
+A data frame with columns \code{a} and \code{b}, one row for each element of
+\code{species}, holding \code{NA} where no relationship is known.
+}
+\description{
+A length axis needs an allometric weight-length relationship for every line
+on the plot. The species take theirs from their species parameters and the
+resource takes its from \code{\link[=resource_params]{resource_params()}}, where it defaults to the
+equivalent spherical diameter (see \code{\link[=resource_length_params]{resource_length_params()}}). Anything else
+— the "Total" row, for instance — has none, and is reported as \code{NA} so that
+the caller can leave it out.
+}
+\keyword{internal}
diff --git a/man/resource_length_defaults.Rd b/man/resource_length_defaults.Rd
new file mode 100644
index 000000000..c87561b82
--- /dev/null
+++ b/man/resource_length_defaults.Rd
@@ -0,0 +1,34 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/setResource.R
+\name{resource_length_defaults}
+\alias{resource_length_defaults}
+\title{Default weight-length parameters for the resource}
+\format{
+A list with entries \code{a} and \code{b}.
+}
+\usage{
+resource_length_defaults
+}
+\description{
+The resource is a composite of everything from bacteria to
+macrozooplankton, so it has no taxonomic length-weight relationship. The
+default is the geometric one that plankton ecology uses instead: the
+\strong{equivalent spherical diameter} of an organism with the density of water,
+\deqn{w = \frac{\pi}{6} l^3,}
+with \eqn{w} in grams and \eqn{l} in centimetres. On a mizer size grid this
+puts the smallest resource sizes at a fraction of a micrometre and a
+milligram organism at about a millimetre, which is the right order for
+bacteria and copepods respectively.
+}
+\details{
+Note that this is a different convention from the one the species use: a fish
+of a given weight is longer than a sphere of the same weight, by a factor
+\eqn{(a_{fish}/a_{resource})^{-1/3}}, about 3.7 for the mizer default
+\code{a = 0.01}. That difference is real rather than an artefact — a 1 mg copepod
+really is shorter than a 1 mg fish larva — but it does mean the resource and
+the species sit on the plot at their own conventions.
+}
+\seealso{
+\code{\link[=resource_params]{resource_params()}}
+}
+\keyword{internal}
diff --git a/man/resource_length_params.Rd b/man/resource_length_params.Rd
new file mode 100644
index 000000000..d235f44a7
--- /dev/null
+++ b/man/resource_length_params.Rd
@@ -0,0 +1,20 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/setResource.R
+\name{resource_length_params}
+\alias{resource_length_params}
+\title{The weight-length parameters of the resource}
+\usage{
+resource_length_params(params)
+}
+\arguments{
+\item{params}{A MizerParams object.}
+}
+\value{
+A list with entries \code{a} and \code{b}.
+}
+\description{
+Reads \code{a} and \code{b} from \code{\link[=resource_params]{resource_params()}}, falling back to
+\link{resource_length_defaults} for a model that does not set them — which is
+every model built before these parameters existed.
+}
+\keyword{internal}
diff --git a/man/resource_params.Rd b/man/resource_params.Rd
index 3fde8a5ad..77d0fb125 100644
--- a/man/resource_params.Rd
+++ b/man/resource_params.Rd
@@ -48,6 +48,17 @@ created: \deqn{N_R(w) = \kappa\, w^{-\lambda}.}{N_R(w) = \kappa w^{-\lambda}.}
 Unlike the carrying capacity, however, the initial resource abundance is
 \strong{not} updated when you subsequently change \code{kappa} (or call \code{\link[=setResource]{setResource()}}).
 
+The resource parameters \code{a} and \code{b} give the allometric weight-length
+relationship \eqn{w = a l^b} of the resource, with \eqn{w} in grams and
+\eqn{l} in centimetres. They feed none of the rates; they exist so that the
+resource can be shown on the length-based plots (\code{size_axis = "l"}) alongside
+the species. They default to the equivalent spherical diameter of an organism
+with the density of water, \eqn{a = \pi/6} and \eqn{b = 3}, which is the
+convention plankton ecology uses for a composite of many taxa. This is a
+different convention from the one the species use, so the resource and the
+species each sit on the length axis at their own; see
+\link{resource_length_defaults}.
+
 Assigning to \code{resource_params} only rebuilds the size-dependent resource rate
 and capacity arrays from these scalars (leaving any arrays you have set
 manually untouched). Changing \code{lambda} also recalculates any \code{q} and \code{gamma}
diff --git a/tests/testthat/test-ArrayResourceBySize-class.R b/tests/testthat/test-ArrayResourceBySize-class.R
index 033b52125..2dd215e3f 100644
--- a/tests/testthat/test-ArrayResourceBySize-class.R
+++ b/tests/testthat/test-ArrayResourceBySize-class.R
@@ -223,8 +223,9 @@ test_that("animate dispatches on ArrayTimeByResourceBySize", {
     expect_length(frames(animate(n_resource_small,
                                  tlim = c(times[2], times[3]))), 2)
 
-    expect_error(animate(n_resource_small, size_axis = "l"),
-                 "length axis is not available")
+    # A length axis is now available, using the resource weight-length
+    # parameters rather than a species'
+    expect_s3_class(animate(n_resource_small, size_axis = "l"), "plotly")
 })
 
 test_that("plot() can express a resource density per logarithmic size", {
@@ -246,3 +247,48 @@ test_that("plot() can express a resource density per logarithmic size", {
     expect_error(plot(resource_level(NS_params_small), per_log_size = TRUE),
                  "only applies to an array that holds a density")
 })
+
+test_that("the resource can be plotted against length", {
+    params <- setResource(NS_params_small)
+    resource <- initialNResource(params)
+    rp <- resource_params(params)
+
+    by_weight <- plot(resource, return_data = TRUE)
+    by_length <- plot(resource, size_axis = "l", return_data = TRUE)
+    expect_identical(names(by_length)[[1]], "l")
+    expect_equal(by_length$l, (by_weight$w / rp$a)^(1 / rp$b))
+
+    # It is a density, so the values are converted as well as the axis
+    expect_equal(by_length[[2]],
+                 by_weight[[2]] * rp$b * by_weight$w / by_length$l)
+    p <- plot(resource, size_axis = "l")
+    expect_identical(p$scales$get_scales("x")$name, "Length [cm]")
+    expect_identical(p$scales$get_scales("y")$name, "Number density [1/cm]")
+
+    # llim trims the length axis
+    expect_lt(nrow(plot(resource, size_axis = "l", llim = c(0.1, 1),
+                        return_data = TRUE)),
+              nrow(by_length))
+
+    # A quantity that is not a density keeps its values
+    mort_w <- plot(getResourceMort(params), return_data = TRUE)
+    mort_l <- plot(getResourceMort(params), size_axis = "l",
+                   return_data = TRUE)
+    expect_equal(mort_l[[2]], mort_w[[2]])
+
+    # and animate() no longer refuses a length axis
+    expect_s3_class(animate(NResource(NS_sim_small), size_axis = "l"), "plotly")
+})
+
+test_that("the resource joins the species on a length-axis spectrum", {
+    params <- setResource(NS_params_small)
+    on_l <- plotSpectra(params, size_axis = "l", return_data = TRUE)
+    expect_true("Resource" %in% on_l$Legend)
+    # The resource sits at its own convention, shorter than a fish of the same
+    # weight by the ratio of the two `a` values
+    on_w <- plotSpectra(params, return_data = TRUE)
+    res_w <- on_w[on_w$Legend == "Resource", ]
+    res_l <- on_l[on_l$Legend == "Resource", ]
+    rp <- resource_params(params)
+    expect_equal(res_l$l, (res_w$w / rp$a)^(1 / rp$b))
+})
diff --git a/tests/testthat/test-plots.R b/tests/testthat/test-plots.R
index b09e521c5..00d8e2c82 100644
--- a/tests/testthat/test-plots.R
+++ b/tests/testthat/test-plots.R
@@ -508,10 +508,14 @@ test_that("size-based plots support length axes", {
     expect_true(all(spectra_l_limited$l >= llim[1]))
     expect_true(all(spectra_l_limited$l <= llim[2]))
 
-    spectra_hidden <- plotSpectra(params_len, species = species,
-                                  resource = TRUE, total = TRUE,
-                                  size_axis = "l", return_data = TRUE)
-    expect_false(any(spectra_hidden$Legend %in% c("Resource", "Total")))
+    # The resource has its own weight-length relationship and so appears on a
+    # length axis; the total still does not, because it is a sum over lines
+    # that each have their own.
+    spectra_all <- plotSpectra(params_len, species = species,
+                               resource = TRUE, total = TRUE,
+                               size_axis = "l", return_data = TRUE)
+    expect_true("Resource" %in% spectra_all$Legend)
+    expect_false("Total" %in% spectra_all$Legend)
 
     p <- plotSpectra(params_len, species = species, resource = FALSE,
                      size_axis = "l")
diff --git a/tests/testthat/test-setResource.R b/tests/testthat/test-setResource.R
index 913cae820..bddaf4dfe 100644
--- a/tests/testthat/test-setResource.R
+++ b/tests/testthat/test-setResource.R
@@ -418,3 +418,39 @@ test_that("explicit balancing overrides a frozen complementary array", {
                       balance = FALSE)
     expect_equal(resource_rate(p3), resource_rate(p), ignore_attr = TRUE)
 })
+
+test_that("the resource carries weight-length parameters with an ESD default", {
+    params <- setResource(NS_params_small)
+    rp <- resource_params(params)
+    # The default is the equivalent spherical diameter at the density of water
+    expect_equal(rp$a, pi / 6)
+    expect_equal(rp$b, 3)
+    expect_equal(mizer:::resource_length_params(params), list(a = pi / 6, b = 3))
+
+    # A model that predates the parameters falls back to the same default
+    old <- NS_params_small
+    old@resource_params$a <- NULL
+    old@resource_params$b <- NULL
+    expect_equal(mizer:::resource_length_params(old), list(a = pi / 6, b = 3))
+
+    # A value the user set is kept, not overwritten by a later setResource()
+    resource_params(params)$a <- 0.02
+    resource_params(params)$b <- 2.9
+    expect_equal(resource_params(setResource(params))$a, 0.02)
+    expect_equal(mizer:::resource_length_params(params), list(a = 0.02, b = 2.9))
+
+    # and is validated
+    expect_error({resource_params(params)$a <- -1; params}, "not greater than 0")
+    expect_error({resource_params(params)$b <- "3"; params}, "not a number")
+})
+
+test_that("the ESD default puts the resource at plausible lengths", {
+    # A sanity check on the convention rather than on the code: the smallest
+    # resource sizes should be micrometres and a milligram organism about a
+    # millimetre.
+    esd <- function(w) (w / (pi / 6))^(1 / 3)
+    expect_equal(esd(1e-12) * 1e4, 1.241, tolerance = 1e-3)   # micrometres
+    expect_equal(esd(1e-3) * 10, 1.241, tolerance = 1e-3)     # millimetres
+    # A fish of the same weight is longer, by a fixed factor
+    expect_equal(((1e-3 / 0.01)^(1 / 3)) / esd(1e-3), 3.741, tolerance = 1e-3)
+})
diff --git a/vignettes/cheatsheet-analysis-and-plotting.Rmd b/vignettes/cheatsheet-analysis-and-plotting.Rmd
index 99f8cce76..f3caf46ba 100644
--- a/vignettes/cheatsheet-analysis-and-plotting.Rmd
+++ b/vignettes/cheatsheet-analysis-and-plotting.Rmd
@@ -369,9 +369,13 @@ plotSpectra(sim, species = c("Cod", "Herring"), resource = FALSE)
 plotSpectra(sim, biomass = TRUE, size_axis = "l")     # biomass density against length
 ```
 
-**The resource is dropped from a length axis.** The resource spectrum has no
-  length-weight relationship, so `plotSpectra(params, size_axis = "l")` shows the
-  species only. Use `size_axis = "w"` to see the resource.
+**The resource has its own length convention.** It is a composite of many taxa,
+so instead of a taxonomic weight-length relationship it uses the equivalent
+spherical diameter of an organism with the density of water (`a = pi/6`,
+`b = 3`, in [`resource_params()`](../reference/resource_params.html)). It therefore appears on a length axis, but
+measured differently from the fish: a fish of a given weight is about 3.7 times
+longer than a sphere of that weight. That gap at the resource-consumer boundary
+is real biology, not an artefact.
 
 ### Which density a spectrum plot shows