[baseballyama]

Summary

The preview drew cartesian charts with a taller plot area than LibreOffice and put line labels on top of their markers, so labels that collide in the real chart (a bar's inEnd value label against a secondary-axis line label) looked separate in the preview. The automatic layout now follows LibreOffice's chart2 rules.

Motivation

A deck generator reviews preview PNGs to fix overlapping chart labels. On a combo chart (columns + a secondary-axis line, bottom legend, 12 pt labels) LibreOffice shows 13,150/51.0%, 14,020/52.0% and 15,640/53.4% overlapping; the preview showed none of them.

Measured plot areas (axis spines, 1280 px wide) for four charts of the same deck:

chart	frame top–bottom	LibreOffice plot	preview before	preview after
combo, legend b	182–614	199–547	190–570	200–545
column, legend b	197–653	214–585	204–609	215–583
line, legend b	211–571	226–499	219–526	227–502
bar, no legend	211–648	220–638	219–621	220–620

Line label glyph rows above a 7 pt marker with a 12 pt label: LibreOffice 349–359, before 357–368 (on the marker), after 349–359.

The inset comes from LibreOffice's source (chart2/source/view/main/ChartView.cxx): constPageLayoutDistancePercentage = 0.02 applied to each side of the diagram space.

The "after" column above is from the first revision, which also reserved a bottom legend box with 1.85 mm gaps (VLegend.cxx) and half a value-label line above the plot. Those two rules come from a newer LibreOffice than the fidelity gate's reference (24.2): against it they moved the legend and plot top away and regressed three sample slides (31-2, 37-1, 40-1). They are dropped; the legend strip and plot top stay as on main. With the remaining changes every slide is within the gate's tolerance (31-2 0.8371, 37-1 0.8997, 40-1 0.7828 against the CI ground truth).

The reference desktop app could not be used as ground truth here: its scripted PDF export timed out on this machine, so the numbers are LibreOffice only.

Changes

• Bar, column, line, area and combo charts with automatic layout: 2% of the frame width / height per side instead of a fixed 8 px.
• Column-style charts size the category-label row from the label font (was a fixed 18 px, too short for 12 pt labels).
• Line labels at t / b keep their line box 1 mm clear of the marker (was a fixed 5 px / 13 px offset from the point).
• Unchanged: pie / doughnut / radar / scatter frames, manual inner plot-area layouts, horizontal gutters for axis labels (still fixed widths), legend placement and its strip.

Testing

• New test/preview-chart-auto-layout.test.ts covers the insets, the font-sized category row, and line label clearance.
• Updated the hard-coded plot geometry in preview-chart-axis-controls, preview-chart-fill-opacity and preview-chart-inner-layout to the new layout.
• npx vitest run test/preview- passes (1155 tests).
• CI's fidelity job compares against LibreOffice and is the gate for the chart samples.

Breaking changes

None in the API. Chart previews render with a slightly smaller plot area.

Checklist

• I have read CLAUDE.md and followed the project's conventions.
• I have added or updated tests for the change.
• I have added or updated documentation where user-visible behavior changed.
• If this is a breaking change, I have added a changeset / CHANGELOG entry and
  flagged it above.

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