Git-Heat-Map¹ does something similar to this. It's not open source but I found it easy re-implement the core algorithm² as a learning exercise in React.
Git-truck is running in a web interface, even though it’s self hosted. But rendering all repos definitely sounds interesting. I had an idea to create a repo, which would basically be a monorepo, where i would register other repositories as submodules. Sadly git-truck does not seem to like that.