Filtering time period

[cognos05]

Hi,
I have a crosstab report like the below one

SalesMeasure       2014/Mar  2014/Apr  2014/May   2014/Jun    2014/Jul    2014/Aug   2014/Sep         2014/Oct   2014/Nov        2014/Dec   2015/Jan       2015/Feb

ProductA                 0                0                  0                  0                0            15               34                    0               67                 56              54                  12


Assume this product is only introduced in 2014/Aug.

I want to draw a line chart starting with my data item only from 2014 Aug till current Date

So I have to find the no on the month in which there was a sale started for the last 12 months and then subtract that by 12.

LastPeriods((MuN for currentMonth that is 2015/Feb), (12  Minus no on the month in which there was a sale started))

I want the expression to be know for getting this value no on the month in which there was a sale started

In the above example it would be (12-6) 6 (ie) 2014/Aug on which the sale started and my line chart will have x axis from 2014Aug to 2015 Feb

Thanks,
Nithya