What is the dimension you are using in the bar chart? Is it something like YearMonth, formatted as Month only?
I wouldn't expect all the bars to change, but if you select e.g. February, your set expression will compare February and January of 2015 with March to December 2014.
While selecting Jan, the February bar should show Feb 2014 value.
So basically the set expression tells QV to list the sales values of the YearMonth one year back in time of the max yearmonth selected.
Is this what you intend to show?