Average of Month Over Month Difference

[Misti Warshaw]

I created a Month Over Month calculation in a bar chart as Sum([Total Cash Flow]) THEN [Value] - First([Value]) OVER (NavigatePeriod([Axis.X],0,-1)) - 1 as [Delta]. This is a custom expression on the Y-axis, not a calculated column. Now I need to calculate an overall average of those Deltas for the selected period and display that number asa calculated value in a text field. The period will change based on user filter selections and can be a single month or multiple months. The math seems simple: Sum of the Deltas for all selected months/# of months but I'm stuck. Any ideas