You can use a table calculation to get the prior 8 months of sales on the same view. In that case, you'll probably want to use a table calc filter on the month so that way the results of the prior 8 months of sales don't change - a regular filter would be processed in the data source and would hide the needed data. I set this up in the attached.
Jonathan - thank you for the quick response. This is much closer to what I am looking for but not quite there. Is it possible to just get the single value of the field 8 months ago instead of the cumulative amount? Could I use a formula like: "WINDOW_SUM(SUM([Sales]),-7,0)" minus "WINDOW_SUM(SUM([Sales]),-6,0)" to get that single amount?
Just make the window calculation start and end value the same (-7).
or maybe this is a bit cleaner:
Even easier: LOOKUP(SUM([Sales]),-8) would get you the value from 8 months ago.
Thank you both! I will give these a try.