A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
I also would find this feature useful (to show increasing costs (negative on the spreadsheet) as a rising sparkline). Similar to toggling left-to-right, there should be a toggle to flip the sparkline over top-to-bottom!
A workaround is to have a separate row (or column) of data that inverts the values (to positive) (e.g. B1=-A1), and create a sparkline of that.