Thank you for mentioning the aesthetics in graphviz. It's not in any sense optimal; we chose a collection of heuristics meant to get somewhere close to diagrams made by hand (with tools). Some areas can clearly be improved. Even the initial ranking and cycle breaking is not always quite right. Possibly some of the middle stages need to be more aware of groups of nodes. How to draw sets of natural looking curves around obstacles in diagrams still seems to be an open problem, do we even know what to optimize? It's natural to ask if machine learning algorithms will get there first.