Pointer, but that filters everything with a matching z value

For hovering a multi-series line chart, we don’t want to just highlight the current point; we want the highlight the entire line. I expect it’d be pretty easy to extend the current pointer interaction so that instead of just selecting a single index i, it filters the index to everything that has the same z value. Then you could apply this “pointerZ” interaction (name TBD) to a line mark, such that the line closest to the pointer is drawn in a different color.

(We probably also want an “exclude” mode for the pointer interaction to draw things differently when not pointed.)

Start by reading the existing pointer interaction and line-mark implementation. Define how matching z values select an entire line, how the proposed pointerZ interaction should be named and applied, and whether exclude mode belongs in the same change; done means the hovered line is visually distinguished and the intended non-hover behavior is specified.