I've not used a visual programming language and unit is (currently) hugged to death. But, my experience with graphviz's dot syntax would suggest putting a comment on the (textual) line that represents the edge itself:
digraph whatever {
running [ shape = "triangle" label = "program running" ]; # comment on the node itself
stopped;
running -> stopped; # comment about the edge
stopped -> running;
}
I acknowledge, though, that I'm thinking of this as a dsp-style situation, where a node only connects at its boundaries, rather than in the center (say if a node contains code that would link to another as part of an if expression's body).(Also, I'm disappointed that I need to resist the urge to talk about Bob Nystom's visual pdf diff, because it seems really cool but is not as credible as the edge directive above. https://journal.stuffwithstuff.com/2021/07/29/640-pages-in-1... , scroll to 3/4 where it says "Here is what all of the proofreading changes look like:")