Helper to rename channels?

[Fil]

The use case is to apply all the transforms we want to x and y, then rename those channels as px and py before passing them to a pointer transform.

This allows to position the pointer target with the usual transforms without having to make them individually aware of the fact that we want them to operate on the px, py channels.

In https://talk.observablehq.com/t/pointer-transforms-with-px-py-on-stacked-bar-charts/8302 I gave a solution that explicitly renames x to px and y to py, but it could be a more generic helper such as channelRename({x: "px", y: "py"}, options)?