Inspired by Bill Turkel's bibliography clusters, I spent the last week playing with GraphViz. Here is a relatedness graph generated by the subject page for my great grandfather:
One big problem with this graph is that it shows too many subjects. Once I add subject categorization, a filtered graph becomes a way of answering interesting questions. Viewing the graph for "cold" and only displaying subjects categorized as "activities" could tell you what sort of things took place on the farm in winter weather, for example.
Another issue is that the graph doesn't effectively display information. Nodes vary based on size and distance. Size is merely a function of the label length, so is meaningless. Distance from the subject is the result of the "relatedness" between the node and the subject, measured by page occurrences. This latter metric is what I'm trying to calculate, but it's not presented very well. I'll probably try to reinforce the relatedness by varying color intensity using the Graphviz color schemes, or suppress the label length by forcing nodes into a fixed size or shape.
Of course, common page links are only one way to relate subjects. It's easier, if less interesting, to graph the direct link between one subject article and another. Given more data, I could also show relationships between users based on the pages/articles/works they'd edited in common, or the relationships between works based on their shared editors. A final thing to explore is the Graphviz ImageMap output format, which apparently results in clickable nodes.
I'll put up a technical post on the subject once I split the dot-generation out into Rails' Builder — currently it's just a mess of string concatenation.