Twelve Month Moving Average in Cognos 8 Report Studio with DMR Model

[ashish53657]

I want to design a chart with moving 12 months' averages .

The line chart is supposed to have 2 components(lines) :

1st line:12 month moving average for total Net Sales of all customers.

2nd line:12 month moving average for total Net Sales of Top 3 customers (out of thousands of customers) only.

I am unable to use moving-average funtion.

Do i need to create a dimension/measure which takes 12 month rolling behaviour into account?

I have used LastPeriods function with some success but can't use it for the second line.

Please refer the image for graph requirement

Please suggest an approach.