Merritt you are asking 2 different questions
the Last() function numbers the table from the bottom up (or from the right if used across the table)
the second issue is a function of the data - if there is no data for the rest of the year what was you expectation - what did you want to see?
If this posts assists in resolving the question, please mark it helpful or as the 'correct answer' if it resolves the question. This will help other users find the same answer/resolution. Thank you.
I approach this issue by creating a calculated field along the lines of
[Date] < DATETRUNC('month', TODAY())
So if Today is 26th July 2018, this calc will return TRUE if the date field you're working with is before 1st July 2018. Then add this to your filter shelf, setting option to true.