After a quick google (I've never played runescape) yes. That's almost exactly what I've got in mind, but I probably won't make it a webpage because I'm getting sick of that sort of thing.
Edit:
(22:40:24) <lsl> If/when I get it working properly I'll integrate it into a site somehow, but to start with I'd like to generate stats for myself
(22:40:40) <lsl> more out of personal interest than anything else
(22:40:58) <lsl> I'm just asking for a few ideas for when I do make it public
(22:41:09) <lsl> should probably state that explicitly

Adding variable domain to feature list

Further edit:
(22:41:56) <Gum> I honestly cant think of anything you haven't listed. Maybe a comparison tool for different colors. Like say I want to compare the prices for vampire and supernova over a month, have the 2 show up in a line graph as separate lines on the same graph
/me slaps self.
/me adds to feature list.