If you have a look at this example.
Here I've loaded all the data into one table, I expect you will use a calendar, so you have to tweak the code/expressions to fit. Perhaps use an index field which counts up each month from the beginning of your time so you don't have to worry about the change from 12 to 1 each year.
With Month 15 selected (as in your example) the expression will show averages (just for A) for each month from the max to Max-6, this can been seen in the right hand chart.
The main chart shows sums by region (for the selection of month 15). The mini chart uses month as its dimension with the same set analysis in the right hand chart which limits its output.
Let me know if you need anything further
AveStraightMiniChart.qvw 153.8 K