How can I sum up the totals of groupings in views? I am trying to sum up my sales in my Commerce store by month.

I have created a view of completed payment transactions that is grouped by payment year (first grouping) and then payment month (second grouping).  The config looks like this:



This gives output look like this:



Now what I'd like to do is sum up each month (the second grouping) to show the total sales for that month.  I think I can use aggregation for this but I'm not sure how to set it up.
Most of the links I found for this kind of thing point towards Views Aggregator Plus.  Apparently it allows you to "aggregate and group by column" (source).