How is your raw data structured ? Please post sample raw data or sample workbook.
Here is one idea.
First of all, you need to reshape the data for Tableau handle the table appropriately.
I added one more column to show act/forecast as monthly value to avoid confusion, and add color of act/forecast period.
act.forecast_SM_10.5.twbx 15.8 KB
Hadn't thought of that- works a dream!