Expr for CurrMonthSales-PrevMonthSales in line chart

[#1]

Hello,

I need to plot month wise sales on Line Chart.

I need to show sales for ex:

For Mar sales should be 'Mar-Feb', for Aug, 'Aug-july'.

I have Month and Slaes data in excel.

Appreciate your quick help on this...

Thanks,

Ananth