Rolling weeks calculation

[#1]

Hello Experts,

I think its a basic query, but still tough as I am new to this. I have a chart which shows sales vs forecast for 52 weeks. However the requirement is this needs to be rolling 52 weeks, as in currently in week 10 for 2011, then the chart should show backwards till the week 11 for 2010, how can I achieve this?? Currently I have this

sum({<TIME_PERIOD_WEEK_NO>}INV_GROSS_REV)

Thanks in Advance

[#2]

Hello,
Even this doesnt seem to have solved the problem I have. I will explain again I need to show a chart with current - 52 weeks of data at any time. which would be like starting from year 2010, weeks 15,16,17,18,19,20...... to Year 2011 Current Week 16. The problem I have is that the chart should show 2 expressions, one for SALES and other for FORECAST. The sales is only till current week but the forecast data is for full year. so in the chart its showing current year forecast full 53 weeks and sales only till week 16. How do i fix it to show both the data only till Current week - 52 weeks?

Any help would be appreciated. Will provide more info if needed.

ANDY