How to add Average to a table like Totals

[Oliviaagrace]

Hi! 

I am trying to do something in Looker that seems like it should be simple. 

I know there is the check box, “Totals” that you can check which would add a row to the end of your data table the totals of each column. I want to do that exact same thing but with an Average, not a total. I have not been able to do it. My table is simple, the first column is dates, and the second column shows the units sold for that week. At the bottom of that column, I want an average to display. 

I am pretty new to Looker, so have been able to do a lot with the basic back-end capabilities, but not versed yet in the SQL / more advanced parts. 

Any ideas?