null, undefined, NaN and -Infinity create holes

[Fil]

Also, ignore infinite values when computing the thresholds.

Demo: https://observablehq.com/@d3/d3-contours-treats-nulls-as-holes-61

closes #49

[mbostock]

This…

• is redundant with low(…)
• treats positive infinity as negative infinity
• uses a magic number when we can handle infinity exactly

The passed-in value should be coerced like this (outside this function, probably in contour as I mentioned in the other comment):

value = value === null ? NaN : +value;
if (isNaN(value)) throw new Error(`invalid value: ${value}`);

Values we read from values should be coerced like this:

v0 = v0 === null ? NaN : +v0;
v1 = v1 === null ? NaN : +v1;

Given that, maybe something like this?

const a = value - v0;
const b = v1 - v0;
const d = isFinite(a) && isFinite(b) ? a / b : Math.sign(a) / Math.sign(b);
point[0] = isNaN(d) ? x : x + d - 0.5;

[mbostock]

Per above comments. Thanks for taking on this work!

[Fil]

New version following your review. Thanks!

[Fil]

Thank you! I don't think this needs an addition to README, it's essentially an extension of the "applicability" (or one could say, a bug fix).