subtract from above row if not null

[kylealpaugh]

Hello,

I have chart where I need to subtract from the previous row but some of the rows are null. Is it possible to use the next non null row instead of the null row.

My current expression.  sum(Sales)-Above(sum(sales)

I would like row 2020-10-12 to use 2020-10-09 instead of the row right above it because the row above is null and does not return a value when used. 

Any help is apricated