Your modifications look great and promising. Have not had a chance to set up profiles to see what it can do yet.
I urge you to submit your modifications as a Pull Request to the Gramps\addon-source repository making your modifications an integral part of the view.
As it stands now, if/when other changes are made to Graph View, if a user has your modification as it is now, your modification will be totally overwritten.
I think it would be better to modify Graphview with a better choice than Gancanvas, which is not supported in Gtk4.
After that, we will be able to implement new features.