fix(xychart): auto-flip point labels to avoid line collision

[DominicBurkart]

Summary

• Adds collision detection for point labels on xychart line plots — when a label's bounding box would intersect an adjacent line segment, the label is automatically flipped to the opposite side of the data point (below instead of above for vertical charts, left instead of right for horizontal charts)
• Fixes the issue visible in PR #1 QA example 6 where the steep descent from PaLM (540) to LLaMA-65B (65) causes the line to pass through the "LLaMA-65B" label

Stacked on #1.

Approach

For each labeled point, the algorithm:

1. Estimates the label's bounding box using fontSize * label.length * 0.7 (same heuristic as bar label renderer)
2. Checks if either adjacent line segment (prev→current, current→next) intersects that box
3. If collision detected, flips the label to the opposite side of the point

Before (PR #1)

Labels always above — line cuts through "LLaMA-65B":

Changes

File	Change
linePlot.ts	Added lineYAtX(), doesSegmentIntersectBox(), shouldFlipLabelVertical(), shouldFlipLabelHorizontal() helpers; modified label loop to use conditional positioning
xyChart.spec.js	3 new Cypress visual regression tests (steep descent, gentle slope, zigzag)
demos/xychart.html	2 new demo cases for manual QA

Test plan

• All 4008 vitest unit tests pass (0 failures)
• Cypress visual regression: steep descent chart — labels should flip below when line would collide
• Cypress visual regression: gentle slope chart — labels should stay above (no false positives)
• Cypress visual regression: zigzag pattern — mix of above/below labels
• Manual QA: open demos/xychart.html and verify the MMLU steep-descent demo

https://claude.ai/code/session_01UCV1QCaRsRtQjRspaVrqUp

[DominicBurkart]

QA Results v2: Label-Line Collision Avoidance

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1. Steep Descent — MMLU Real-World Example

PASS — "LLaMA-65B", "Llama 2 34B", "Mistral 7B", "Phi-3-mini" flipped below their points, away from the descending line. "PaLM" stays above.

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2. Gentle Slope — No Collision (regression check)

PASS — All labels ("Start", "Mid", "End") remain above. No false positives.

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3. Zigzag Pattern — Mixed Collision (previously broken, now fixed)

PASS — Valley points ("Low" at P2, "Bottom" at P4) now correctly flip labels below. Peak points ("High" at P1, "Peak" at P3) stay above.

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Summary

#	Test Case	Labels Position	Result
1	Steep descent (MMLU)	Flipped below on descent	PASS
2	Gentle slope	Stayed above (default)	PASS
3	Zigzag (v1 bug)	Peaks above, valleys below	PASS

Automated tests: 4008/4008 vitest unit tests pass, 0 regressions.

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Generated by Claude Code

[DominicBurkart]

Superseded by mermaid-js#7551