A co-worker today introduced me to ‘iftop’ which is an incredibly handy utility for monitoring current bandwidth utilization through an interface. Its sortof like a simple, command-line version of ntop. It is available in most Debian repositories, and in the RPMForge repositry if you use RHEL/CentOS. More information on its homepage at https://www.ex-parrot.com/~pdw/iftop/
Also, another handy bandwidth monitoring program to run on your server is vnstat. It runs via cron every 5 minutes and can provide some simple historic bandwidth usage graphs. I didn’t see any packages for vnstat, but its the simplest build I’ve ever seen. Just download it and run ‘make && make install’ More information at https://humdi.net/vnstat/
You may be interested in pmGraph, which we just released as open source (https://pmgraph.sourceforge.net). If you try it, please let us know how you get on and how we can improve it.