diagrams.net (formerly draw.io) have an xml format that seems fairly open and they can also read/write a bunch of other formats (see here https://www.diagrams.net/blog/import-formats ) so something that gives you their xml format as a starting point might make sense.

discussion of format: https://stackoverflow.com/questions/59416025/format-of-draw-...

Looks like they have an api too: https://jgraph.github.io/mxgraph/docs/js-api/files/index-txt...