Using draw.io diagrams in GitHub:
The page at www.alderg.com/github.html does the I/O with GitHub and uses draw.io in embed mode for the editing. The page supports the following URL parameters: user, pass, repo, path, ref and action=open. Using action=open, links for immediate diagram editing in GitHub can be created (requires user and pass parameters).
Supported file formats: .png, .svg, .html and .xml (default)