Center 3D subplot titles on their scenes

Author: robertoffmoura

plotly/matplotlylib/renderer.py

@@ -146,6 +146,7 @@ def __init__(self):
         self.scene_ct = 0
         self.current_3d_subplot = None
         self.current_is_3d = False
+        self.all_3d = False
         self.mpl_x_bounds = (0, 1)
         self.mpl_y_bounds = (0, 1)
         self.msg = "Initialized PlotlyRenderer\n"
@@ -289,10 +290,10 @@ def open_figure(self, fig, props):
         )
         self.plotly_fig["layout"].paper_bgcolor = _export_color(props["figbg"])
         self.mpl_x_bounds, self.mpl_y_bounds = mpltools.get_axes_bounds(fig)
-        all_3d = fig.get_axes() and all(
+        self.all_3d = bool(fig.get_axes()) and all(
             getattr(ax, "name", None) == "3d" for ax in fig.get_axes()
         )
-        if all_3d:
+        if self.all_3d:
             has_title = any(
                 getattr(ax, "title", None) and ax.title.get_text()
                 for ax in fig.get_axes()
@@ -1361,6 +1362,11 @@ def draw_title(self, **props):
             self.msg += "          More than one subplot, adding title as annotation\n"
             x_px, y_px = props["mplobj"].get_transform().transform(props["position"])
             x, y = mpltools.display_to_paper(x_px, y_px, self.plotly_fig["layout"])
+            if self.all_3d:
+                # the scene domains of all-3d figures span the horizontal
+                # axes bounds of the figure
+                x0, x1 = self.mpl_x_bounds
+                x = (x_px / self.mpl_fig.bbox.width - x0) / (x1 - x0)
             annotation = go.layout.Annotation(
                 text=props["text"],
                 font=go.layout.annotation.Font(

plotly/matplotlylib/tests/test_renderer.py

@@ -1648,6 +1648,36 @@ def test_plot3d_shorthand_axis_colors():
         assert axis.tickfont.color == "#FF7F0E"
 
 
+@pytest.mark.parametrize("nrows,ncols", [(1, 2), (2, 2)])
+def test_plot3d_subplot_titles_centered_on_their_scenes(nrows, ncols):
+    """In figures of 3D subplots, each title sits at the horizontal
+    position across its scene domain that the matplotlib title has
+    across its axes."""
+    fig = plt.figure(figsize=(4 * ncols, 4 * nrows))
+    axes = []
+    for i in range(nrows * ncols):
+        ax = fig.add_subplot(nrows, ncols, i + 1, projection="3d")
+        ax.plot([0, 1], [0, 1], [0, 1])
+        ax.set_title("title {0}".format(i))
+        axes.append(ax)
+
+    plotly_fig = tls.mpl_to_plotly(fig)
+
+    layout = plotly_fig.layout
+    scenes = [layout.scene] + [
+        layout["scene{0}".format(i + 1)] for i in range(1, len(axes))
+    ]
+    for ax, scene, annotation in zip(axes, scenes, layout.annotations):
+        x_px, _ = ax.title.get_transform().transform(ax.title.get_position())
+        bounds = ax.get_position()
+        mpl_rel_x = (x_px / fig.bbox.width - bounds.x0) / bounds.width
+        rel_x = (annotation.x - scene.domain.x[0]) / (
+            scene.domain.x[1] - scene.domain.x[0]
+        )
+        assert annotation.text == ax.get_title()
+        assert rel_x == pytest.approx(mpl_rel_x, abs=1e-6)
+
+
 def _add_scatter3d(ax):
     ax.scatter([0, 1], [0, 1], [0, 1])