For web things, I would recommend the combination of dagre (https://github.com/cpettitt/dagre), which will calculate node and edge positions, and d3.js (https://d3js.org/), which you can use to layout your nodes as an SVG.

I did some work on something a little bit similar for an EU research project, which you are welcome to take a look at. It's probably not directly applicable, but it may give you some thoughts about what does and doesn't work as a UI: https://github.com/cse-bristol/process-model/ https://tools.smartsteep.eu/process-model/?name=actionplanni...