8900AX2400 snmp OID for received WAN traffic speed

[nightcustard]

I have to confess that all I know about snmp is what I've read today, so apologies if this is a stupid question:
Is there an snmp OID corresponding to the received WAN traffic speed (kB/sec or equivalent)?
I've found an OID corresponding to total bytes received on ptm0.1 (1.3.6.1.2.1.2.2.1.10.9), so I could poll this at intervals and calculate the speed - is this how the graphing clients work?
Many thanks in advance!

8900AX2400 running firmware 2.52.d27 VDSL2 interface