Month over Month data in chart

[tschullo]

I have a capacity/usage data set and am trying to show the monthly usage in a chart while also showing the delta alongside it.  I tried above() (and before () in pivot) and that works nicely until I make a selection of one or more months, especially disjointed months.

I then tried using set analysis to try to set the MonYr field to the previous month like so:

Sum({<MonYr = {$(=AddMonths(MonYr,-1))} >} USAGE)

but it always returns the current month's usage, I'm guessing because it can't ignore the MonYr dimension for that row.
I even tried Sum({1<MonYr = {$(=AddMonths(MonYr,-1))} >} USAGE), but no dice.

I'm sure this has been done before.  Can someone please help?
I need it to be dynamic so that filters can be applied and the values are recalculated accordingly.

[Almen]

I did something similar in my pivot table, maybe it works on chart table too?

It looks like this:

Before(SUM(AGGR(COUNT(YourValue),
Dim1,[DateValue.autoCalendar.YearMonth])))