Investigation summary

Validated #561 against main. The root cause is the truthy check on tooltipX / tooltipY in HTMLTooltip.svelte (lines 102–103): when a datum has x === 0 or y === 0, the ternary falls through to 0px instead of the projected position. Datum selection is unaffected; only CSS positioning is wrong.

The ?? 0 fix in this PR is correct and minimal. Hidden-tooltip behavior is unchanged.

Approving the fix. One gap before merge: please add regression tests in packages/svelteplot/tests/htmltooltip.test.svelte.ts (existing harness covers datum selection but not position styles).

Requested tests

1. y = 0 — data [{ x: 50, y: 0 }], plot width=200 height=100, domains [0, 100]. Hover near the projected point and assert tooltip.style.top is not '0px' and is close to 100 (bottom of plot).

2. x = 0 with centered domain — data [{ x: 0, y: 50 }], x.domain = [-50, 50]. Hover at center and assert tooltip.style.left is not '0px' and is close to 100.

These fail on main and pass with this fix — good TDD coverage for the falsy-zero case.