cross-facet brushing

[Fil]

Plot.brush({sync: true}) synchronizes the brush selection across all facets — when the user brushes in one facet, the same rectangle appears in all panes and reactive marks update across all facets.

The filter function's facet arguments are now optional: omitting fx or fy (or passing undefined) skips the facet check, selecting points across all values of that facet.

This resolves @allisonhorst 's suggestions #2364 (comment) and #2364 (comment).

I'm keeping this as a separate PR for now (against #2360) to make the logic more apparent. Ideally if we merge this I would migrate the tests and examples to brushX on parallel time series, rather a 2D brush on penguins.

Disclaimer: I used @claude to generate the tests.

[Fil]

An issue: what if a facet's value is undefined? I might need to revise this PR so that the facet filter happens with an explicit object {fx: undefined}, which would mean "filter on the undefined facet", vs. {} or null which would mean “ignore facet when filtering”.

[allisonhorst]

Nice! This works as expected for me. One thought I had was what happens here if we add an option for free x/y scales in faceted charts.

[allisonhorst]

Not specific to the cross-facet brushing: one thing I noticed is that the opaque brush border is a bit distracting with the line mark. I think it'd be rare that a person would want to brush in the x and y direction on a line chart (instead of just brushing over time in the x direction, for example), but sharing in any case.

brush-border.mp4

[Fil]

It's not an opaque border: the discontinuities you're seeing on the edges of the brush are because the line is segmented between elements that are outside (context), and elements that are inside (focus) — and they're not connected. A way to avoid this is to draw a ghost line (complete), with the focus line on top (and no "context" line). I don't think we can do better by default.

[Fil]

what happens here if we add an option for free x/y scales in faceted charts

Yes, it will be tricky. Most probably it just doesn't make sense to support syncing in that case?

Ref. #8

[allisonhorst]

the discontinuities you're seeing on the edges of the brush are because the line is segmented between elements that are outside (context), and elements that are inside (focus)

Ah this makes sense!

[Fil]

For a line chart I wouldn't recommend using context like we do for individual dots, but instead would draw a (complete, greyed out) "ghost" line, and the selected line on top.