Graph Summarization Support

While looking through the graphviz filter issues I found the feature request Summarize Graphs? and figured this is something we should take into consideration while working on sub graph support. I think that this is an important topic when thinking of graphs and modelling (like erd). Here is a good article on that topic: http://odetocode.com/blogs/scott/archive/2008/05/19/visual-designers-don...

I don't have the requirement currently and other issues need attention first but I wanted to create this one so we can collect further information on that topic.