I've never really paid attention to PR (although apparently my blog is sitting at 4 without trying anything, which is kind of neat), but I'd really like to see

I've never really paid attention to PR (although apparently my blog is sitting at 4 without trying anything, which is kind of neat), but I'd really like to see is a graph of Page Rank over time. Does such a service exist? I doubt it would be easy to do so for any arbitrary page, but even a way to add my site to it and check it occasionally (perhaps get email alerts when the status changes?) would be cool.