Since Set Analysis is calculated once for the entire chart, you won't be able to do what you need with only one expression. You'd need to create a chart without dimensions and 12 expressions, and for each one use set analysis to set the min and max MonthYears you want to acumulate for it . For instance,
If you select June 2013:
June 2013: Jun 13 + May 13 + Apr 13 + Mar 13 + Feb 13 + Jan 13 + Dec 12 + Nov 12 + Sep 12 + Aug 12 + Jul12
May 2013 : May 13 + Apr 13 + Mar 13 + Feb 13 + Jan 13 + Dec 12 + Nov 12 + Sep 12 + Aug 12 + Jul12 + Jun 12
Apr 13 : Apr 13 + Mar 13 + Feb 13 + Jan 13 + Dec 12 + Nov 12 + Sep 12 + Aug 12 + Jul12 + Jun12 + May 12
and so on...
Hope it helps
Jun-2013 = values of the selected product in these dates
And May-2013 = ?
If it's 05/2013+04/2013+03/2013+02/2013+01/2013+12/2012+11/2012+10/2012+09/2012+08/2012+07/2012+06/2012 then you want is what I call a 12-month rolling total. In which case I wish you good luck doing this in any way that is not discussed in the document I referred to.