We have done something like this that we basically used the power of stored procedures. Basically, you know how many months you'll be dealing w/ so that setup could be static, then within the stored proc, bring back each row as a calculation as it looks like there is no raw data on the dashboard. Not the prettiest way of doing things, but you have complete control within the SP and can then add a metric whenever.

You will need to incorporate some logic for grouping/sorting.