chart: Add appear motion to Plot and every chart

[madcodelife]

Summary

Charts now draw their data in the first time they are painted, over 1000 ms on Chart.js' default easeOutQuart. The mainstream defaults are ECharts, Chart.js and Highcharts at 1000 ms, ApexCharts at 800 ms and Recharts at 1500 ms. The design system's enter curve, an exponential ease-out, made the draw-in read as a flash, so the appear uses its own curve. Each effect follows what the mainstream libraries do (checked against their sources):

Chart	Appear	Path cache
Line / Area / Candlestick / Sankey	Revealed from the left under a content mask (ECharts, Highcharts, Recharts)	Shapes unchanged, cache stays warm
Bar	All bars grow out of the zero line together (Chart.js, Recharts, ApexCharts); value labels fade with their bar	No cache (quads)
Pie	The ring sweeps clockwise (Chart.js animateRotate, Recharts); leader-line labels fade in at the end	Uncached only while sweeping
Radar	Series grow out of the center (Chart.js, Recharts)	No cache

Axes, grid lines and tick labels are there from the first frame, and the tooltip waits until the data is whole. New data paints in place, so charts fed live quotes never replay. Reduced motion skips the appear.

The chart story gets a Replay button that re-keys the gallery so every chart draws in again.

Public API

gpui-base

pub struct PlotAppear                                  // this frame's appear of a plot
impl PlotAppear {
    pub fn complete() -> Self                          // a finished appear
    pub fn progress(&self) -> f32                      // eased 0..=1 for the whole plot
    pub fn is_appearing(&self) -> bool                 // still running
    pub fn staggered(&self, index: usize, count: usize, spread: f32) -> f32 // per-mark progress, starts spread over the first `spread`
}
impl PlotMotion {
    pub fn with_appear(self, appear: Transition) -> Self // how data draws in (zero by default)
    pub fn appear(&self) -> &Transition
}
trait Plot {
    fn interactive(&self) -> bool { true }             // hover/tooltip, now separate from `id`
    fn appear(&mut self, appear: PlotAppear, window: &mut Window, cx: &mut App) {} // receive the appear before hover/paint
    fn appear_generation(&self) -> Option<u64> { None } // `Some` opts in; a new value replays
}

gpui-component

On LineChart, AreaChart, BarChart, PieChart, RadarChart, CandlestickChart and SankeyChart:

pub fn appear(self, appear: bool) -> Self       // opt out, e.g. charts in list rows that remount on scroll
pub fn appear_key(self, key: impl Hash) -> Self // replay the appear when the key changes (symbol, period)

The theme projects PlotMotion::with_appear(1000 ms, easeOutQuart).

Breaking Changes

None at the source level. The three new Plot methods all have defaults (interactive → true, appear → no-op, appear_generation → None), so an existing hand-written plot compiles and behaves exactly as before: with an id it tracks hover as it did, and it tracks no appear and asks for no frames. PlotMotion gains a field behind a builder, and its default adds no motion.

Behavior changes worth knowing:

• Built-in charts draw in by default. Opt out per chart with .appear(false).
• interactive(false) charts now return an id. Plot::id on them is Some (it was None), and Plot::interactive is false. They still get no hitbox, but they now keep element state under that id. Like interactive charts, siblings built at one site without .id(...) share the appear state and path caches. That stays correct, but it rebuilds geometry more often.

  let chart = LineChart::new(data).interactive(false);
- assert!(Plot::id(&chart).is_none());
+ assert!(Plot::id(&chart).is_some());
+ assert!(!Plot::interactive(&chart));

Performance

• Once the appear is done, it costs nothing extra: no mask, no frame requests, and PlotAppear returns early without sampling the easing. The per-frame state key is allocation-free, and the theme is borrowed rather than cloned.

• A chart with .appear(false) (or a hand-written plot that doesn't opt in) tracks no state and requests no frames. Covered by tests.

• Timing harness (not committed) on 100 line + 100 bar + 50 pie charts, 200 points each, --release, avg CPU per frame:

  	during appear	steady
  main	~420 ms	~411 ms
  this branch	~374 ms	~404 ms

  No regression. Per type during the appear: line 140 ms, bar 139 ms, pie 0.86 ms (vs 0.32 ms steady, from re-tessellating while sweeping).

Test plan

• cargo test -p gpui-base -p gpui-component -p gpui-kit
• New tests: appear timing over the theme duration, no frames once finished, reduced motion, opt-out requests no frames, generation replays, stagger math, interactive(false) keeps its id
• cargo clippy -p gpui-base -p gpui-component --lib --tests
• Ran the chart story: Replay restarts the appear. Mid-frame shows the area revealing from the left and pies sweeping; after 1 s everything is whole.