Edit/Add the service monitoring rule for “CPU utilization for simple devices”.
You will get a second graph with a line for each core.
Core 1 is the Management Plane
Core 2 is the Data Plane.
A multi-core Data Plane is still reported as 1 Core. There is no SNMP counter for each single DP core as far as I know. However a Palo Alto does a pretty good job in levelling the load over its available DP cores.