I've got a screenscraper pulling in all rankings of a friend's books on fictionwise four times a day, and dropping that into a db. Which then gets graphed by a most wonderous php->xml->swf graphing library. Dude.
Can't recommend that graphing library enough. I'm not paying for it, so clicking on the generated graph will take you to their site. Long live shareware! If I ever have a client that wants me to do something like that, I will more than happily advise them how to fork out the money. :) It's not that much, in the scheme of things.