Voronoi initializer (for pointer interactions)

[Fil]

new description!

This PR chiefly addresses #1622, making the voronoi mark work with the tip option. A notable performance issue in the modified usStateCapitalsVoronoi test is that the clip path is created anew each time we rerender the mark (this is addressed separately, in #1624).

A secondary issue (#1858, making voronoi work with exclusive facets) was first addressed in an ad hoc fashion (in 71c49a2). After this review comment suggesting that this should be centralized, I reverted the change in 70f2352, then used the new exclusiveFacets transform (introduced in #1648) in 972558c et sq.

Closes #1622
Closes #1858.

[mbostock]

I couldn’t resist trying the Voronoi initializer idea… I dunno if we need to make similar changes to the Delaunay marks? I think maybe it doesn’t make sense there, though, because there’s a not a 1:1 correspondence between the data and the geometry.

Screen.Recording.2023-05-23.at.7.01.59.PM.mov

[Fil]

the voronoi cell now depends on i and fi (with a test showing facet: exclude)

[mbostock]

This seems mostly fine… but there’s something that bothers me. I guess it’s that we need to create nested/faceted channels because we can’t assume that the facets are exclusive. #1648 And our workaround abuses how channels are intended to be represented (parallel to the data). And it’s especially funny that the Voronoi mark handles this but the stack transform doesn’t—the stack transform is far more frequently used!

I would rather fix the non-exclusive facet problem generally rather than apply the workaround in this PR. I think we probably need a redux of the approach in #1069. Ideally, it’s like a transform wrapper that says “this transform/initializer needs exclusive facets”, and it expands the faceted index and channels as needed to meet the constraint that the facets are exclusive, and the downstream code therefore doesn’t need any changing.

[Fil]

It works! Applying the exclusiveFacets to the voronoi mark only for now (I saw that it didn't work with the voronoiMesh).

[Fil]

In the documentation there is a voronoi mark with the centroid initializer, and the reindexation logic fails in that case.

[Fil]

I'm not sure how to apply exclusiveFacets properly. At least we now have test cases. Rather than a transform as it is now, I wonder if it could be a (private?) option flag instead, that asks Plot to apply this before any other transform. It would feel less "ad hoc" for the voronoi marks.

[Fil]

I've updated the description of this PR. I'd like to find a cleaner way to apply the exclusiveFacets transform (DONE)