You are lucky. See this recent post here: https://forum.mendix.com/link/questions/88124
You can testdrive the app here: https://grapheditor.mxapps.io/
Yes! Not with Rappid or GoJS specific but with D3.
For one of our projects we needed something similar, so we started with a small Rappid widget. However, the scope changed and such we needed other features, thus we made a new D3 widget. This is really easy with the boilerplate found at https://github.com/Finaps/boilerplate-mendix-react-widget. You can clone the boilerplate, then run 'npm install' and 'npm install -S d3'. After that you can just import D3 (or if you prefer any other graphing library) in your widget.
If you have questions on how to proceed with the boilerplate, please send me a message! I'll be glad to help!