As already mentioned, MRTG is an ideal tool for this type of job.

One limitation of MRTG is, however, that it only allows you to display two sets of data in one graph. Something else that you may wish to look at is RRDTool (written by the same person as MRTG). RRDTool is much more flexible than MRTG, and there is a handy Perl module available.

I have worked quite a bit with both, although my preference is RRDTool, because of its greater flexibility. For monitoring network traffic, my approach was to examine the values in /proc/net/dev on a regular basis (on a RH Linux box - well, actually around 230 linux boxes ;). I've also used it for other stuff such as monitoring the number of users on a network, monitoring latency, etc. I'd thoroughly recommended it.

cheers,
Darren :)