Hi!
To use MRTG o PRTG or any snmp based graph tool your switch must have a L3 interface , you need a interface vlan .
Config sample:
Your mrtg/prtg must be on the same segment of vlan1 address.
interface Vlan1
192.168.0.1 255.255.255.0
!
snmp-server community mrtg2008 RO