You should probably mention the tool you are using for this: RRDTool. It's the same tool I use to track the number of items in my email inbox (http://www.jgc.org/blog/2007/06/measuring-my-inbox-depth.htm...) which I publish on my web site.

I've used RRDTool many other places, and it's immensely useful: http://oss.oetiker.ch/rrdtool/