Fix point size scaling when filtering to single value

[lhawkman27]

Summary

• Fix computeSizeScale to return midpoint size instead of minimum when all data points share the same value (range === 0)
• Detect noSizeRange in transformProps and collapse legend to a static swatch with accurate singleValueColor
• Add single-value fallback rendering in both Legend and MultiLegend components
• Tighten singleValueColor type from number[] to RGBAColor, extract shared helpers, and add comprehensive test coverage

Focus Score

9/10 — All changes are tightly focused on the single bug: point size scaling when filtering reduces data to one unique value. The type tightening and helper extraction are direct refinements to the same code paths.

Detailed Summary of Each File Changed

• src/utils/colors.ts — computeSizeScale: when range === 0, return Math.round((startSize + endSize) / 2) instead of startSize. Null/NaN guard still returns startSize.
• src/transformProps.ts — Detect noSizeRange by comparing sortedValues[0] to sortedValues[last]. Collapse legend startSize/endSize to the midpoint when true. Compute singleValueColor via metricColorScale so the legend swatch matches the rendered point color. Add legendName to SizeLegend for fallback display.
• src/components/Legend.tsx — Change singleValueColor type from number[] to RGBAColor on SizeLegend. Extract renderSingleValueRow helper. Use it for both standalone size legend and combined metric+size single-value fallback. Remove all as RGBAColor casts.
• src/components/MultiLegend.tsx — Extract singleValueSizeRow variable to deduplicate two identical 10-line JSX blocks. Remove as RGBAColor casts. Used in both combined metric+size and standalone size legend branches.
• test/components/Legend.test.tsx — Add 3 tests: singleValueColor swatch rendering, fillColor fallback, combined metric+size single-value fallback.
• test/components/MultiLegend.test.tsx — Add GraduatedIcons/CategorySizeGrid mocks and 5 tests: simple entry with collapsed size, combined metric+size swatch, standalone size swatch, singleValueColor usage, graduated icons positive control.
• test/utils/colors.test.ts — Update computeSizeScale tests for midpoint behavior when range is 0.
• test/utils/percentileBounds.test.ts — Update expected size values to reflect midpoint instead of startSize.

Test Plan

• Run full plugin test suite: npx jest --no-coverage plugins/geoset-map-chart/test/ — all 590 tests should pass
• In a dashboard with a map chart using dynamic point sizing, apply a filter that reduces to a single data point — verify the point renders at a visible mid-range size (should hold it's shape)
• Verify the legend shows a static swatch with correct color instead of empty/graduated icons
• Remove the filter to restore multiple points — verify graduated icons and normal size scaling return
• Test with combined metric+size coloring (same column for color and size) — verify swatch color matches the point
• Test with separate metric and size columns — verify legend renders correctly

🤖 Generated with Claude Code

[ethanbienstock]

LGTM!