iptraf and rrdtool   ()

http://www.taedium.com/rrd-iptraf/
One of a network administrator's most important tasks is to monitor network activity. mrtg is by far the most popular tool to do this. But sometimes looking at straight network traffic isn't enough — sometimes you need to profile the traffic more precisely to figure out what's happening. To do this, we'll use iptraf and rrdtool.

http://people.ee.ethz.ch/~oetiker/webtools/rrdtool/tutorial/rrdtutorial.html
rrd tool tutorial

http://cebu.mozcom.com/riker/iptraf/