Generating extra channels with the group transform?

[mbostock]

Should it be possible to use the channels option with the group transform (and bin transform)? That way you could generate and group an extra channel that is unknown to the mark. I think currently the channels option simply gets propagated, so you can’t reference an extra channel as a group output.

[mbostock]

I found myself initially wanting this again here to address the “sort by something that’s not a channel” use case:

https://talk.observablehq.com/t/observable-plot-cell-how-to-order-y-axis-by-a-specific-field/8120/2

But I ended up using the data sort option and a reduce function, which felt pretty good. In that sense this is closely related to #867. So I think doing something smarter with “extra” channels in the group and bin transforms would be helpful.