I think you could get your accumulated value using rangesum() function together with above() chart inter record function.
I created some sample data using
LOAD *, Month(Date) as Month, year(Date) as Year;
Date(makedate(2011)+recno()-1) as Date,
ceil(RAND()*100 ) as Value
Then I created a line chart with dimensions Month and Year to match what I think is shown in your image.
The expression to accumulate the last 12 months then looks like:
The important part here is the rangesum(above(total sum(Value),0,12)) function that will sum the last 12 Values in a table, for each line (last values means the 12 lines above the current line, thus we use the above() function).
Since I believe you are showing dimensions in the order Month, Year in your chart, but we want to group by Year, Month, I added an additional advanced aggregation to correct for this.
You will notice that the acculated line will have a steep slope for the first 12 Month, since we accumulate the last 12 Month and there is not enough data for these month, so Jan 2011 will only accumulate 1 month, Feb 2012 2 months etc.
See also attached.
comm46239.qvw 152.2 K