Pivot table with each column having the previous columns value deducted from it
I have a pivot table with a dimension of 'companyname', column of 'month' and a measure of SUM(number_of_sales).
I want to change the measure expression so each column shows the number of sales minus the number of sales in the previous month, so sort if the opposite of a cumulative, the idea being to show how the number of sales per month has increased month on month.
How would i go about this, i'm not even sure where to begin?
