Merge pull request #424 from neilcawse/fix/trendlines

Charts: read, write and draw trendlines

# Conflicts:
#	docs/gaps.md

docs/gaps.md

@@ -44,9 +44,9 @@ No alpha blockers: every core workflow passes the [alpha gate](roadmap.md#alpha-
 
 ### 4. Charts lose type and options on round trip; shallow chart model (F, FF)
 - **Missing:** kinds that survive XLSX (stock, combo, histogram, box, waterfall, funnel, treemap,
-  sunburst), axis scaling, label options, trendlines, error bars, per-point formatting.
+  sunburst), axis scaling, label options, error bars, per-point formatting.
 - **Evidence:** `Chart` holds only title, legend position, two flags and axis titles
-  (`crates/model/src/features.rs`); PRs #158, #161 open; charts can't be resized (#190). Smooth lines (#62) and Switch Row/Column (#116) landed.
+  (`crates/model/src/features.rs`); PRs #158, #161 open; charts can't be resized (#190). Smooth lines (#62), Switch Row/Column (#116) and trendlines (read, written, drawn) landed.
 - **Impact:** charts are in most business workbooks.
 - **Estimate:** 25–35 h for beta (40–60 h full). [chart-parity.md](chart-parity.md)
 
@@ -160,6 +160,7 @@ No alpha blockers: every core workflow passes the [alpha gate](roadmap.md#alpha-
 |---|---|---|
 | 2026-10-11 | minor | §6: iterative calculation for circular references landed (Maximum Iterations, Maximum Change) |
 | 2026-10-11 | minor | §15: trim-reference operators (`A1:.E10`, `A1.:E10`, `A1.:.E10`) parse and evaluate |
+| 2026-10-11 | minor | §4: trendlines (six types, forecast, equation and R²) read, written and drawn |
 | 2026-10-11 | minor | §16: comment navigation, Show Comments, Show All Notes and Convert to Comments landed (#281) |
 | 2026-10-11 | minor | §7: `.xls` (BIFF8) import of values, formulas and number formats (#121) |
 | 2026-10-11 | minor | §3: XLSX pass-through keeps custom XML, external links, connections, query tables, printer settings, controls and OLE objects (#330, #324) |