I think you are right on the snmp counter, but I don't see how to change the default for graphing bit rate on a link. I just have the 'Graph bit rate' box checked for the link and using SNMP for mastering type. The SNMP profile for the device is SNMP v2. Also, the monitoring interval is 1 min and setting it to less than 60 sec doesn't make a difference. It seems to use a 1 min avg no matter what I set the polling interval to, although I didn't try going more than 60 sec.
As a work around, I created a custom probe for the device using, rate(diff64(oid("18.104.22.168.22.214.171.124.1.10.5")))*8/1000000. This seems to be working correctly, but is a lot of work to do for every interface that may go over 500 Mbps.