Previous Year's Month data in Webi Report

Hi Gurus,

I have a requirement like below in webi.

There are prompts 'Year' and 'Month'

The report will populate '#Calls' for the year and month given in prompt.
For EX: Nov 2012
        #Calls: 23

Now I require '#Calls' for the same month in previous year's data
EX: Nov 2011
    #Calls: ??

Please let me know how this can achieved in webi.

Any help is appreciated.

Thanks,

Shamu