Can we add a check to the final year to see if it has 12 months of data points?

If we can...

If the year has fewer than 12 months of data - display a message:
Your final year does not include 12 months of data. Please update to a year with complete data. To view mid-year progress, select an earlier year and run a "Savings Report".

Open to tweaking on the text.

If we cannot, maybe a message below the line that is always present:
An analysis is intended to be run with complete years only. To see mid-year progress, please run a report for an earlier year and then run a "Savings Report".