fix: render points sharing one date

[huytdps13400]

Summary

• keep matching points inside a zero-duration X range
• center the degenerate range on the canvas
• draw every value at the shared X coordinate so Skia receives a visible vertical path

Root cause

When the first and last timestamps are equal, X normalization divides by zero. The resulting NaN causes getPointsInRange to discard every point before path creation. Even after normalizing the shared timestamp, the one-pixel sampler selects only the first point, leaving Skia with a move command but no drawable segment.

The patch maps the shared timestamp to the center of the range, rejects non-matching timestamps from an explicit zero-duration range, and preserves all same-date values at that X coordinate. The existing behavior for different dates that merely floor to one canvas pixel remains unchanged.

Validation

Using the repository Node.js version (22.20.0):

• yarn test --runInBand — 5/5 tests pass
• yarn typecheck
• yarn lint — 0 errors; one existing no-shadow warning in AnimatedLineGraph.tsx
• yarn prepare
• git diff --check

Fixes #86

[riteshshukla04]

3 year old issue :)