smooth_run gives every segment the same number of sub-segments (cap / n, 1..100) so long
series are smoothed end to end within the point cap, and clamps the Catmull-Rom curve to
the plot box. List "smooth" in chart.addElement's params doc, drop the smooth_demo
example, and use the test sheet's own name in series refs.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Add support for Excel's "smoothed line" chart style. The smooth flag flows
model -> resolve -> chart render, persists through XLSX (<c:smooth val="1"/>),
and is toggled via the chart.addElement "smooth" element.
Rendering interpolates each gap-free run with a uniform Catmull-Rom spline
(0.5 tangent scale, 100 sub-segments per data segment), so the curve passes
through every data point and rounds the vertices — matching Excel's observed
behaviour. Long runs are still decimated before smoothing.
- model: Series.smooth (#[serde(default)], so old workbooks load unchanged)
- chart: SeriesData.smooth; smooth_run + catmull in render; wired through
resolve and the sanitize() copy; draw_runs takes the flag (line + scatter)
- engine: chart.addElement element="smooth"
- xlsx: read/write <c:smooth val> instead of hard-coding val="0"
- tests: smooth_line_adds_interpolated_points_and_keeps_endpoints,
smooth_line_splits_at_gaps, smooth_line_setting_round_trips
- examples/smooth_demo.rs: generates a straight+smooth side-by-side workbook
cargo xtask ci: all 7 steps pass.
