Reserve Matplotlib legend layout space [mpl compatibility]

[sselvakumaran]

Summary

• reserve plot padding for anchored legends from their measured title + entry geometry
• match Matplotlib's font-relative legend defaults (borderpad=.4, labelspacing=.5, borderaxespad=.5, 2em handles and column spacing)
• keep SVG, raster, and browser legend positioning/sizing consistent
• preserve literal $ labels such as the price legend's $ 2.00

This is stacked on #248 (agent/line-legend-gallery-compat).

Exact Matplotlib gallery comparisons

Outside top legend: horizontal_barchart_distribution

The previous legend touched the axes border/first row. The new measured reservation leaves the same ~0.5-font border-to-axes gap as Matplotlib.

The clipped left category labels (Question appearing as estion) are intentionally not claimed as fixed here: the exact latest-heads union shows #240 does not resolve that canvas-gutter defect. A separate general gutter follow-up is required for full visual completion.

Titled scatter legends: scatter_with_legend

The previous static layout truncated Ranking to Ra... and undersized title/entry bounds. The new boxes use the title and every entry when measuring width and height.

Verification

• 547 passed, 65 skipped in tests/pyplot
• focused legend/rc/grid tests: 22 passed
• TypeScript typecheck and browser bundle build pass
• exact Matplotlib 3.11 gallery scripts rerendered at 100 DPI
• 88-example execution totals unchanged: 55 paired scripts / 79 paired figures; no execution-count regression
• ruff check and git diff --check pass

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch agent/legend-layout-gallery-compat

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[codspeed-hq]

Merging this PR will degrade performance by 15.87%

❌ 1 regressed benchmark
✅ 101 untouched benchmarks
⏩ 2 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	test_pyramid_compose	9.3 ms	11 ms	-15.87%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing agent/legend-layout-gallery-compat (12d0cb9) with agent/line-legend-gallery-compat (cf7cae4)}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩