cumulative average by month

Hi,

ActualDate - Amount

10/22/16 $1000

10/23/16 $1100

10/25/16 $ 900

11/01/16 $1000

11/11/16 $1100

11/21/16 $1200

12/01/16 $400

12/28/16 $500

I want to do avarage monthly, which is 1000 for Oct/2016, 1100 for Nov/2016, 450 for Dec/2016. Then accumulate current month average and averages of the months before current month. The visual should be like:

I googled a lot, but almost scenaria is discussing doing average such as totalAmount/monthAmount.

Thanks in advance.