Cumulative value reset monthly

I have a list of of orders that are posted on differrent days - some orders are on the same days too.

I have added these up so that I can view 'total sales per day',. 

This works well, but now I want to view it in a cumulative fashion where it resets at the start of each month. 

I have some troubles gettingt his to work - I have tried several suggestions but it either throws an error or doesn't reset at the start of each month. 

So I should be able to sum up the values displayed in "Total Sales Per Day" in a cumulative way, and then I want it to be reset when there's a new month. 

Thanks!