fix: handle graph points sharing one pixel

[huytdps13400]

Summary

• Handle graph ranges where every data point resolves to the same canvas pixel.
• Avoid dividing by a zero pixel span and producing a NaN graph-data index.
• Add regression coverage using the dates and wide range shape from Cannot read property 'date' of undefined when range is too large #95.

Root cause

The path sampler derives an array index from the relative pixel position. With a very wide X range, flooring can map both the first and last graph points to the same pixel. The resulting endX - startX value is zero, so the computed index becomes NaN and the code reads graphData[NaN].date.

A zero-width pixel span has only one drawable pixel. The sampler now selects index zero for that degenerate case while leaving the existing interpolation unchanged for every non-zero span.

Validation

• yarn typecheck
• yarn lint (0 errors; existing no-shadow warning in AnimatedLineGraph.tsx)
• yarn test --runInBand
• yarn prepare

The regression test failed on main with Cannot read properties of undefined (reading date), then passed with a finite path after the guard.

Fixes #95.

[chrispader]

Thanks for the PR. LGTM!