If you are thinking about visual language you have to think about many constructions that collectively give you user experience, including available basic construction blocks (how looping, branching works etc), structures, lambdas, every language construction, how you give hints to user what is possible and what is wrong and additionally how to do this real 2 way transformation of code and graph.

Luna infers types and displays colors according to them. We use algebraic data types, lazy evaluation, pattern matching and purely functional paradigm which suits graph visualization really well. We fine tune performance by allowing lazy data visualisation and many, many more things. I don't know how we can even visualize standard OO abstractions to be usable and pleasant to work in the visual form. It does NOT mean we did not think about it. We did for a long time.

Sure you can utilize Luna gui to visualize anything, including python code, but it would be insanely hard to deliver similar functionalities to what Luna offers out of the box (which is available by careful design of both representations).

I feel thin answer is very vague, but I hope I put a little light on how complex this task is. Did I answered your question (at last partially)?