That plug in, checks Rx and Tx counters on a router/switch, they can be asked via SNMP (as luca said) knowing the OID. Once it got the values, substracts the previous values obtained to the new ones, and divides everything by the time period between the checks.
PD: if can try gnuplot too (nice & easy graph tool).
PD2: excuse my poor english. :S
Edited Tue Aug 23 2005, 07:03AM ]