Trending Analysis in QV

[#1]

Hi All,

I'm pretty new to Qlikview and have been playing around with set analysis but cannot seem to get the logic on how to implement trending. Here's what I'm trying to achieve:

I have the following fields 

1. Reporting Period - Displays all months ex. Jan-2011, Feb-2011.....Dec-2011. This is a calculated field from a time dimension table by concatenating the Month and Year fields.
2. Month_Num - Numeric field that gives number of current month. Ex Jan-2011 is '1' , Feb-2011 is '2' , etc.
3. Field X - The field on the Y-axis that could be a cost or something like that.

As you can see these are the two sheet objects I have. Now what I want is if I select a particular month, based on my current selection the logic should look back six months and display all the corresponding data.

For example:

If I select Jan-2011, I should get data only for Jan-2011, if I select Mar-2011 I should get data for Jan, Feb and March-2011. Similarly if I select Oct-2011, I should get data from May-2011-Oct-2011 i.e. data for the last six months including current selected month.