Wouldn't it also be useful to do a syllable/word count/some other textual analysis thing per post per time? I mean, things may be going well, and the posts more infrequent (or, erm, less frequent. Not sure which of those constructs I like better. Are they the same? I wouldn't be convinced.) but do they increase in length/complication?

I once did a word count vs time, and a friend of mine turned it into a nice graphic. Being a flash-and-trousers types, she removed the legends, but hey, it looks nice, and the trend is correct. Or was when the graph was made.

http://www.stray-toaster.co.uk/jpgs/graph.png

A clarity index v time might be good as well, as maybe you become more (oe less) coherent during different moods. Not that I am implying I have ever found your writings incoherent, oh no, but it is a nice (if vague and cuddly) metric. Can I stop digging now?