arealine mark

[mbostock]

Alternative to #2133 and #2405. Fixes #1866 #2111. Needs documentation…

[Fil]

I like that the render code is much easier to read than in #2133; however I think I liked the {line: true} option better, because it still feels to me that (visually!) this is an area mark with a different style, rather than a different mark. (But that's easy to do in a future enhancement by checking the option in the areaY function and branching to AreaLine.)

[mbostock]

This could be supported on all marks…

[Fil]

Once we support color, we'll often want to use this over stroke or fill, because it's less cognitive work—so it should "just work" everywhere.

I feel it can be a bit tricky though, because it might not make sense for all marks to set both stroke and fill. For instance, I don't think we'd want to set a fill on a line (it's rarely a good idea), or a link (it doesn't apply, so better stick with fill: "none"), but maybe on an arrow (it can apply if there is a non-zero bend), although I would rarely do it.

In this sense "color" should mean "the base color for the mark" rather than "a shorthand for stroke and fill".

[mbostock]

I think the simpler definition of shorthand is easier to understand (and worse is better) and we shouldn’t try to define a “base color” separately for each mark.

I can remove the color shorthand from this PR if you think it’s a bad idea and we can discuss it later.

[Fil]

Yes, please remove it, I think it might set the wrong expectation

[mbostock]

For what it’s worth we already support a color option on the following marks: axis, grid, auto, crosshair, bollinger. The definition I am using here feels consistent with that existing usage.

[Fil]

Thanks, I like this API