subtracting in cdef

[MikeF1]

I have a graph that shows pages printed count for a printer.
I also show a min value for that on each graph. So one can see how many pages were printed in that time frame.
I am trying to subtract the current/max value from the min value to actually display the total pages printed, for that graph.
My cdef does not seem to work.
cdef=CURRENT_DATA_SOURCE,CURRENT_DS_MINIMUM_VALUE,-

Debug shows this
CDEF:cdefd=a,0,- \

Why woud CURRENT_DS_MINIMUM_VALUE come back 0?
Im showing min as not being 0.

I have also tried, with no luck.
cdef=a:LAST,a:MIN,-