you'll want to use set analysis within your expression for this.
Have a look at this for a good explanation of it.
if you need further help, post up an example app and can take a look
hope that helps
First of all, How can you select last 12 months. suppose date is 6 june 2014,
how will you select data from 7 june 2013 to 6 june 2014 ??
Second , if selection is made , then that will affect other reports, which anyone wouldn't demand.
If on every row of a chart, we need to calculate according to date in that row, then this solution will not work.
Do you agree with this ?