Ability to get selected facet in selection handlers

[Salik Syed]

I have a faceted heatmap. I'd like to store the users selection within a particular facet using a box selection. The selection callback takes in encodings and returns intervals -- after several hours exploring, it seems like there is no way to get the actual facet that the selection lies in.

{
    name: "box",
    select: { type: "interval", encodings: ["x", "y"] },
    resolve: "union",
  };

This would be a very useful and important feature for us -- happy to help contribute if someone can guide me on where to start.

[Dominik Moritz]

Selections are in Vega-Lite. Can you file an issue there instead? We might still need to make a change in Vega but I think it makes sense to discuss it there.