This is a good case for using a calculated dimension with the 'Suppress When Value Is Null' option ticked.
Essentially you are saying that you only want to show 12 results which are the current MonthYear and the previous 11 MonthYears. This is the same as saying display all MonthYear values where the MonthYear is greater than or equal to the current MonthYear-12.
To use this in a calculated dimension:
Firstly I assume data to be the following:
I created a straight table using a calculated dimension and Sum(Value) as the expression.
The calculated dimension was:
= If(MonthYear>= Addmonths(Now(),-12) ,MonthYear,)
This is basically saying that if the value of MonthYear is greater than or equal to the value of the current MonthYear-12 then display the MonthYear, otherwise hide the values. It is critical that you tick the 'Suppress When Value Is Null' option for this to work.
The resulting straight table will show:
i.e. showing the last 12 months of data based on the date on which you open the application. Note June and July 2014 data is not displayed.
You can format the MonthYear to show MMM-YYYY, in which case adapt the calculated dimension to:
which will then display
If you have 0 values against a month you will have to set up a master calendar to avoid gaps in your data.
Hope that helps.