In general I'm curious how people handle managing diffs in the workflow over time. I've found when working with Microsoft SSIS, that I end up preferring something in code where changes are obvious.
Why visual programming tools don't (at least in documentation) address version control is an interesting question, given how important that is in programming generally, and the fact that those tools are often focused on enterprise audiences.
So there's a distinction there between the graph, and layout of the graph ie x,y coordinates of nodes, and routing of edges. What would version control track? This is a non trivial problem imo. Edit; specifically, storing a graph in some canonical form so that trivial changes would not create massive 'false positives' in the diff. For example changing an edge indicatin an entire sub graph was added or removed. The problem is similar to xml tree diffing.
This is why I think versioning in these tools is pretty much non existent, imo of course.