Why is there no source editor that allows graphical annotations?

I wonder all the time - why have source code and annotations to be separated? Wouldn't it make sense to integrate notekeeping and source into a single file? For example, why can't I add graphical notes, like snapshots from documentaion, to my C source file?

Some situations where this would be valuable: - I am implementing something from a scientific publication. I want to keep reference and implementation in the same place. - Embedded programming. There may be an external circuit that is controlled by my code. Both implement a function and should not be isolated.