opacity legend 2

[Fil]

Upgrade the color legend, to

• support for display-p3 colors
• have a checkerboard
• support dark mode

a test showing the 3 2 new features:

[mbostock]

I find the checkerboard ugly, but more importantly, it doesn’t match what you see in the chart itself (since charts don’t have a checkerboard background). The use of a checkerboard pattern therefore feels like a technical cue; the reader must apply their knowledge of the checkerboard convention from other applications to say “a ha, this is an opacity encoding.” Often I don’t think the user cares about the distinction between opacity and color, since in practice due to compositing an opacity encoding is a color encoding.

Of course I like that the opacity legend supports CSS4 and dark mode, so we should do that, but I’m going to remove the rest.