How do I ask about LAST MONTH?

[#1]

I have a graph with 3 lines - Budget, Forecast, Paid (actuals) - see chart below.  For Budget and Forecast it shows the whole of the project, by month.  For Paid, I want to show just the last month I have full transactions for - "LAST MONTH".

THIS FORMULA

=If(CanonMonthYear <= Today(), RangeSum(Above(Sum(TrxnAmount), 0, RowNo())))

...It works -

   - it shows me actuals.

   - the bars stop at THIS MONTH. (April in the chart below)

I want it to stop at LAST MONTH.  How can I change "Today" to Last month?  I want the bars below to stop at March.

[sunny_talwar · Accepted solution]

May be like this

=If(CanonMonthYear < MonthStart(Today()), RangeSum(Above(Sum(TrxnAmount), 0, RowNo())))

[#3]

THANKS!  it worked!