Why not use MRTG, RRDTOOL or a combination of both. There are tons of add ons for each of these and you can monitor all sorts of things like: Traffic, CPU usage, Disk space, CPU temp and much more..
you can check them out here :
http://people.ee.ethz.ch/~oetiker/webtools/mrtg/
http://people.ee.ethz.ch/~oetiker/webtools/rrdtool/