Reserve canvas room for categorical tick labels [mpl compatibility]

[sselvakumaran]

What

Reserve enough left canvas space for authored and categorical y-axis tick labels instead of allowing long labels to be clipped by the export boundary.

This is intentionally stacked on #240 because that PR supplies the autoscale/default-margin foundation. I also rendered it together with #248 to verify the reported gallery case. #240 alone does not fix the clipped Question 1–Question 6 labels.

Exact gallery comparison

Columns are Matplotlib 3.11 reference / current combined PR heads before this fix / #248 plus this fix.

The after image keeps every category label fully inside the canvas. The legend-border spacing is handled separately by #254.

Validation

• PYTHONPATH=python /Users/srimans/dev/reviz/.venv/bin/python -m pytest tests/pyplot -q
  • 555 passed, 65 skipped
• focused gutter tests
  • long category labels reserve at least 100 px and export with positive SVG x coordinates
  • short category labels retain the core default gutter
• exact scoped-gallery render of horizontal_barchart_distribution.py on the Fix line and legend gallery compatibility [mpl compatibility] #248 stack

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🧪 Generate unit tests (beta)

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[codspeed-hq]

Merging this PR will not alter performance

✅ 102 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing agent/gallery-canvas-gutters (adff265) with agent/fix-pyplot-autoscale-margins (3b3c2ab)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[sselvakumaran]

legend is still cut off a bit on the left.