Full Metal Jacket is a general purpose, pure dataflow language, supporting recursion, higher order functions, strong typing, type inference, and macros. It's entirely graphical, but it is possible to call Lisp.
It has a very simple, regular, syntax, with only vertices, edges, constants, and enclosures. No special syntax is required for conditions, iteration, or new type definitions. It is homoiconic, though I haven't yet made full use of that.
It is an alternative model of computation, potentially enabling different parts of an algorithm to run concurrently.
It is still under development.
It's implemented in Lisp, so the underlying data are Lisp data structures. Programs are stored as S-expressions, are not particularly easy to read or safe to modify.
I ended up doing roughly the same thing. The graph is based on text, stored in arrays in memory. I just convert to JSON to store on disk. You wouldn't want to mess with the JSON, but technically you could if you didn't make any mistakes.
You can't actually run the programs in parallel though, right? The structure is there but you'd need something on a chip to take advantage of it?
What you've built sounds very interesting, but this part is a problem you may need to solve before programmers will embrace it. If I can't use git with it, I can't develop any non-trivial software with it.
Technically, what git shows you is already a visual diff ;) and they like to draw the diff tree instead of describing it textually because it makes a lot more sense.
(a1 a2 (b1 b2))