fix: prevent negative cost values and improve label visibility in evals chart

[roomote]

This PR fixes two issues with the evals chart on the website:

Issues Fixed

1. Negative cost values: The X-axis domain calculation could result in negative values when the minimum cost was less than $5. Added Math.max(0, ...) to ensure the cost axis always starts at $0 or higher.

2. Truncated labels: Labels for data points at the top of the chart were being cut off due to insufficient top margin. Increased the top margin from 0 to 20 pixels to provide adequate space for labels.

Changes Made

• Modified apps/web-roo-code/src/app/evals/plot.tsx:
  • Updated X-axis domain calculation to prevent negative values
  • Increased chart top margin from 0 to 20 pixels

Testing

The changes have been verified to:

• Ensure cost axis never shows negative values
• Provide sufficient space for labels at the top of the chart
• Maintain all existing functionality

Fixes requested via Slack mention.

---

Important

Fixes negative cost values and improves label visibility in evals/plot.tsx by adjusting X-axis domain and chart margin.

• Behavior:
  • Prevents negative cost values on X-axis in plot.tsx by using Math.max(0, ...).
  • Increases top margin from 0 to 20 pixels in plot.tsx to prevent label truncation.
• Testing:
  • Verified cost axis starts at $0 or higher.
  • Ensured labels at the top of the chart are fully visible.
  • Confirmed no existing functionality is broken.

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[github-actions]

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[roomote]

Reviewing my own code because apparently I trust no one, not even myself.

[roomote]

Good fix for preventing negative costs! I notice the Y-axis already uses this same pattern on line 194, so this improves consistency.

One thought: could we also consider adding a small right margin (maybe 10-20px) to prevent potential label truncation on the right edge, similar to what we did for the top margin?

[roomote]

The value 5 appears to be used as a rounding factor here and in other domain calculations. Would it make sense to extract this to a named constant like AXIS_ROUNDING_FACTOR or add a comment explaining why 5 was chosen? This would make the intent clearer for future maintenance.