Adding custom icons to editor margin with a plugin?

Hi,

I'd like to add a custom icon to the margin in the editor next to a line of code, just not quite sure which ui class I should be looking at to extend. The end goal is to be able to add an icon to the marin based on the code to indicate something about the code, same as the red breakpoint icon and others. Any tips on direction would be appreciated.

Thx!