Graph annotations matter a lot for diffing I think. I think this is actually a good space to explore as it is a hard problem, and would require some serious UX research on how to handle this.

I feel that the one-dimensionality of text makes merging and reasoning about diffs simple, because they are easy to represent side by side. To diff something like an image, or a graph, you probably want to see them on top of each other, however that makes it harder to see the whole.