This is probably a better topic for the Network Management forum, but basically you'll need to ascertain the OID for each port you want to monitor (each port has its own OID), then configure MRTG to monitor these OIDs.
The newer versions of MRTG allow you to use port names. I recommend using this over the trying to define the OID instance. This has two advantages: it is easier to see in the MRTG.CFG file, and if the OID instance changes (which I've seen this happen on routers when upgrading IOSs) your graphs are mapped to the port name, and will continue graphing the correct port.
I am running MRTG 2.9.21 on an NT 4.0 system, but it should be the same for *NIX systems.
You'll have to determine what the port name is called via SNMP get. Here's a sanitized MRTG example from one of my routers for a FastEthernet port. You MUST have the backslash in front of the port name or it won't work.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...