CDEF : DATA1-DATA2 ?

[toniofbb]

hi,

I need some help. I don't know how to make an operation between two datas from the same graph.
ex:
Data1 = Mem Used
Data2 = Mem Total

Data3 = Data2-Data1

But I don't know exactly how to do it with the CDEF functions because I don't know how to give more than 1 parameter to the CDEF functions : "Current_data_source"