subtract one datasource from another

[GamBit]

I have a RAS-Router and graph the active syncronous ppp calls with cacti.
I also graph the asyncronous ppp calls with cacti.

My problem is, that the value of the asyncronous PPP calls includes the syncrouns ppp calls.

Is it possible to tell cacti to subtract the syncronous ppp from the asyncronous ppp calls?

[j]

Similarly, I want to divide one snmp datasource by another and graph the result. Is that possible?
What would be the steps in brief?