ordinal opacity

[mbostock]

Fixes #2284.

[mbostock]

Hmm, this still crashes if you try to have a threshold opacity scale, which is a separate but related bug:

Plot.cellX(d3.range(10), {fill: "red", opacity: Plot.identity}).plot({opacity: {type: "threshold", legend: true, domain: [1, 2, 3], range: [0.2, 0.4, 0.6, 0.8]}})

But also it crashes if you try to display an ordinal opacity scale with a ramp legend, which should be supported (because we can display ordinal color scales with a ramp legend, too):

Plot.cellX(d3.range(10), {fill: "red", opacity: Plot.identity}).plot({opacity: {type: "ordinal", legend: "ramp"}})

[mbostock]

This has some merge conflicts with #2416…

[mbostock]

Unfortunately it looks like the color-mix(…) technique of #2419 doesn’t actually work (at least here) in part because the canvas element the ramp legend is using is detached, and hence currentColor isn’t available. (And more generally it won’t be reliable to have a color scale that references currentColor if we’re baking the colors into an image for the legend…)

[mbostock]

Resurrected the SVG filter technique from #2416. 👍

[mbostock]

I decided it’s easier for the ramp & swatches legends to understand opacity scales, rather than trying to have the opacity scale masquerade as a color scale.