Graphically show difference between two fields over a period of time
i have a table which reports on the amount of tape libraries beeing used per month.
Fields: Library_Name ; Scratch ; Month
Data : Lib1 ; 34 ; June
Lib1 ; 36 ;July
I would a calculation field which shows me that currently there was a growth by +2 from last month. I somehow cant get my head around the calculation.