The code for writing is here: https://github.com/jgraph/drawio/blob/master/src/main/webapp... Reading is here: https://github.com/jgraph/drawio/blob/master/src/main/webapp... We're using the tEXt chunk.

PNG allows for sidecar data, so you can just encode your "source" there.

I'm guessing you can just append arbitrary data to a png file since it has eof markers in its file structure.

PNG is structured in "chunks" of data of different types, and you can add your own tool-specific types. IIRC there is even a way to indicate what other tools should do with a chunk type they don't recognise (keep it untouched or throw it away).

I exported a draw.io png and they do encode the diagram information with private data chunks.