IIRC, I started with the Mondrian API, but switched to Roassal API to expose the functionality and extension points that I needed. Maybe wasn't necessary, I'm not sure, but Mondrian felt like a "middle man."

So in the new universe I could try again to formulate it with Mondrian or otherwise switch to... is there a counterpart to Roassal?

Here is the meat of the code for the visualization:

It's not much. Separately there is some code for defining the shapes and colours of the nodes, and filtering which edges should be considered for the tree layout.