Reverse moving average

Hi,

We are on BI4.1 SP3. we have a requirement to do running average. The webi report is in a date descending order. e.g. I can get the running average in asending order

Date                Prod     Running AvgProd

01/07/2014      100      100

02/07/2014       46       (100+46)/2=73

03/07/2014       60       (100+46+60)/3=68.7

but if the date changing to descedning order, how can we keep the same running avgprod

e.g.

Date               Prod     Running AvgProd

03/07/2014     60          68.7

02/07/2014     46          73    

01/07/2014     100        100

Could anyone please help.

Thanks,