Problem

Our naive approaches to crosshair cursors https://matplotlib.org/devdocs/gallery/widgets/cursor.html have the disadvantage that the crosshair is drawn on the figure. This means an expensive redraw of the the figure for every cursor position change and results in a laggy behavior.

Proposed solution

We could grow an API on Canvas to draw a crosshair with GUI native lines. That should be relatively easy. We only need the size of the Axes-under-cursor in figure coordinates to determine the length of the crosshair.

The goal would be to add a crosshair tool in addtion to Pan and Zoom.

This tool should be much easier to use than a Cursor widget and much more efficient.