fix: prevent _decimalPlaces infinite loop when multiplier overflows to Infinity

[JSap0914]

Summary

_decimalPlaces(x) in src/helpers/helpers.math.ts can enter an infinite loop for certain finite numbers — specifically, values smaller than approximately 1e-305.

Root Cause

The algorithm multiplies e by 10 each iteration, looking for a power of 10 where Math.round(x * e) / e === x. For very small numbers, the required power of 10 exceeds Number.MAX_VALUE (~1.8e308). When e *= 10 overflows to Infinity:

• x * Infinity = Infinity
• Math.round(Infinity) / Infinity = NaN
• NaN !== x is always true → infinite loop

Fix

A single guard inside the while loop:

if (!isFinite(e)) {
  return;
}

This returns undefined when e overflows, consistent with the function's existing behavior for non-finite inputs (!isFiniteNumber(x) path).

Affected values

Any finite x where the loop needs e > Number.MAX_VALUE to converge — in practice, numbers smaller than approximately 1e-305. Examples: 1e-306, 1e-307, 1e-308, Number.MIN_VALUE.

Impact on callers

_decimalPlaces is called in scale.linearbase.js during tick generation. Returning undefined there falls through to Math.max(undefined, ...) = NaN in the decimalPlaces variable — the same degenerate outcome as passing a non-finite axis range, which is already an unsupported input. The infinite loop (current behavior) is strictly worse than returning undefined.

Testing

New assertions added to the existing it('should get the correct number of decimal places') block — no existing tests were modified.

This fix was developed with AI assistance.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.