Pivot Table/Chart - Weekly Counts that roll up into average weekly counts in month, QTR

I have a large data set that I count the number of entries per week. I would like to have a pivot chart that has those weekly counts, but can roll up to an AVERAGE volume for the month, quarter and year.

So if for January
WW1 1,200
WW2 1,000
WW3 800
WW4 2,000

January Average 1,250