Definitely sounds intriguing, though I don't think I have any
preferences on the matter as of now.
If you haven't before, you should get a look at the online free
version of Quicken ... they have a lot of those sort of things. I
tried it out a while back, but I still prefer to do my regular book
keeping with SimpleD/dsBudget. I'm hooked, haha!
But there's a whole section for "trends" in there, and it sounds
pretty similar to what you're proposing.
Cheers,
~Nate