Plot product of 2 values

[frackers]

I'm trying to plot the product of two values returned from a script.

They are the voltage and current from a wind turbine, I want to use a CDEF (if its possible) to plot power (which is the product of the two).

I've tried using the current_value as one input and refering to the other value as a custom string with the database name and value (somefile.rrd:VAL) and variations thereof but no luck.

Am I going to have to calculate this value before I pass it to the data collection script?

Thanks for any clues...