It's open source, I just haven't linked it in the project (my bad).

https://github.com/lnenad/grafly/

In the upper right there is an import/export button that could be used for this. It's stored in localstorage so you could also dump that to wherever you like.

edit: added link to the repo in the about modal. edit2: added import export of the entire localstorage entry on the bottom of the diagrams(left) panel.