When an x-y plot's axes are sky coordinates in a supported system (e.g., ra/dec) it would be desirable to be able to overplot coordinate grids in other systems on the plot, much as is possible for image displays.

It would be highly desirable for the UI for controlling these layers to have as much in common with the UI for the same functions on images.

(As usual, improvement suggestions from me are food for thought, not instructions to implement anything, until confirmed by Xiuqin and promoted to stories.)