This might be totally useless, but I wanted to join the league of people who use the Google chart API and the data dump to create pretty pictures.
So I took a look at the relative sizes of the "communities" on the site, i.e. the numbers of users who are active in certain tags, and also how much the communities overlap. A user is considered a member of a tag's community if they have at least three questions or answers in the tag. Quality (i.e. post scores) doesn't count; this is solely about the number of users.
Thoughts? Any three-combinations you'd like to see?
"The three C's" as per Koper's request and the three ancients for Pollyanna:
You can see additional graphs as requested by other users below. Note that this is several years old (and I have long stopped updating this).