plot: Paint grids as quads and cache tessellated line and area paths

[huacnlee]

Description

A chart card repaints on every frame it is on screen — a scrolling list moves it — and profiling the Longbridge AI chat on iOS (Time Profiler, iPhone 17 Pro simulator, Release) put the chart cards at 12% of all CPU while visible. Two things dominated:

• Grid::paint stroked every grid line as a path: lyon tessellation per line, and for dashed grids a PathMeasurements + sampling pass first. Grid lines are axis-aligned, so each line (or each dash of one) is now a 1px paint_quad centred on the coordinate — the pixels the 1px stroke covered. Dash segmentation follows the SVG stroke-dasharray rules PathBuilder implements (odd arrays repeat), with unit tests.
• Line::paint / Area::paint tessellated their curves every frame, though the vertices only depend on the projected points relative to the chart origin. New in plot:
  • PathCache — builds a path once at a zero origin and moves its vertices to the frame's origin afterwards;
  • ShapeKey — cache key from the projected points and stroke style;
  • PathCaches — a plot's caches kept in window element state (use_keyed_state), since plots are values rebuilt on every render;
  • Line::paint_cached / Area::paint_cached — opt-in; the plain paint methods are unchanged.

Adoption in a plot:

let caches = PathCaches::for_paint("lines", window, cx);
caches.update(cx, |caches, _| {
    for (ix, line) in lines.iter().enumerate() {
        line.paint_cached(&bounds, caches.slot(ix), window);
    }
});

Measured on the AI chat with several vis-chart cards and a quote card on screen, same scroll script:

share of draw time	before	after
Grid::paint	5.6%	1.1%
lyon tessellation	5.3%	1.1%
chart cards (vis_chart_card)	11.6%	5.3%

StrokeStyle gains Hash/PartialEq/Eq so it can key a cache.

How to Test

• cargo test -p gpui-component --lib plot:: — grid dash segmentation and 1px line boxes; PathCache builds once per key and translates per origin; ShapeKey changes with points and extras.
• Any chart story: grids (solid and dashed), bar, line, area, pie render as before.
• Passed cargo run for story tests related to the changes. (not run — verified in the AI chat on the iOS simulator: dashed grids, bars, pies render as before)

🤖 Generated with Claude Code

https://claude.ai/code/session_01HY6KNBpkTcKkW2PpjrFhji