display of previous data from selected one

[#1]

i wish to display the data of the previous 12 months.

Upon selecting any month-year(as dimension) i must get data on a line/bar/combo chart of the past 12 months.

I have tried rolling 12 months but its not the sum of past 12 months which i want. i want the chart to display the entire graph data from the selected month to past 12 months.

i am having trouble writing the proper set analysis.

my dimension is [month year]  (jun 2016 type) and measure is [Year Salary]

i am writing my set as such : sum({<  [month year]= "  --This is the part am messing up-- " >}[Year Salary])

please help

[#2]

I didn't get that bit. it chooses the date column from database? how?

I replaced that with [monthyear] and it works fine too.

however my doubt is what is the monthfield, quarterfield and yearfield is used for? what are they referring too in the database?