Add note or description to a visualization

Can I add a note to a visualization that will show up when the mouse is hoverd over it (or something similar)? I dont mean tool tip. I want to add a note about lets say how I calculated something or definition of certain terms or some text that will help explain how to read / interpret the visuzalition. 

I dont want to add a text box because that will take up too much space for each visualization. 

Any idea?

Thanks