Hi to everyone

I have this request:

I need to show the difference or variation from two periods (years or months) whether using a Pivot View or a graph, in this is case is better in a Pivot view. 

In the pivot view I will have the amount or results from each month, but I couldn't figure out how to calculate the difference from month to month ("var" column from the image), I hope I explained myself and someone can give me a way to do this and learn from.

I had read the documentation but I haven't found something useful to do this.

Thanks in advance.