`areaY` plot does not correctly render discontinuities

[plmrry]

The Plot.areaY function does not seem to respect null values the same way Plot.area does:

https://observablehq.com/@plmrry/area-y-bug

[mbostock]

The stack transform is likely coercing null to zero. This might be intended… I’ll need to think about this more.

[mbostock]

Although this isn’t explicitly documented, this is functioning (at least partly) as intended: the stack transform, like d3-array methods such as d3.sum, ignores null, NaN, and undefined values when computing the extent of each stacked column. The code can be seen here:

plot/src/transforms/stack.js

Lines 107 to 109 in e0bf09e

	if (y < 0) yn = Y2[i] = (Y1[i] = yn) + y;
	else if (y > 0) yp = Y2[i] = (Y1[i] = yp) + y;
	else Y2[i] = Y1[i] = yp; // NaN or zero

You probably didn’t intend to use the stack transform in this case, and the areaY shorthand probably shouldn’t use the stack transform since it doesn’t make sense in conjunction with indexOf. You can fix your code by setting the y2 channel to opt out of the stack transform:

Plot.plot({
  width,
  height: 200,
  marks: [
    Plot.line(data, { x: (d, i) => i, y: (d) => d }),
    Plot.areaY(data, { y2: Plot.identity, opacity: 0.1 })
  ]
})

I’ll make the areaY shorthand do this by default.