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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
I've been making diagrams using graphviz, but setting the positions and edge curves manually, pulling the nodes and edges out of a list in excel
https://github.com/mathew-j-davis/boxesandarrows
I've been setting the 'waypoints' for more complex curves (curves with many bends) manually (typing the bezier numbers in by hand), while I try and figure out how I want them routed, you're right it's not easy figuring out rules for where edge should go, let alone implementing it