How to write VBA code that waits for the user to select a chart?

I wrote some code that currently only works if a chart is already selected/activated before running the code.
Could you share a VBA example that waits for the user to activate a chart (or a shape object) before proceeding?
Is this an event-handling issue?

I tried assigning my subroutine to a rectangle shape-object on the same worksheet as the chart (poor man's button), but unfortunately this shape becomes the active object (stealing "active" status from the chart), so when it calls the sub, the sub just exits with an error. Would assigning this sub to a User Form button have the same outcome? I haven't experimented with making my own GUIs yet.

Thanks,
Geophysicist