use graphviz layers for interactive display
layerselect attributes of GraphViz it should be possible to write an interactive widget that has a slider showing the different layers of an automaton. This could be used to replay the progression of an algorithm interactively, while preserving the global layout, for instance
I believe this could be implemented by adding some named properties "layer-edges" and "layer-states" which would be vectors of layer numbers, and let
print_dot() know about it. Then we also need some Python functions to set those layer attributes.