While going through the documentation, I noticed that this section of the online docs was broken. I was able to reproduce that locally, and found that I was missing the dot executable, because I installed graphviz with pip instead of my package manager it seems. After installing it with my package manager I did recover the graph.

So I'm thinking dot wasn't accessible on whatever machine was used to build the docs. Note that this is has been the case since version 0.19.2, so the stable version of the documentation is also affected by this.