would like to see your twbx workbook with the data to give you a more specific response -
but in concept there are a couple of considerations - first parameters are the way users input single values into tableau - the parameter on its own does nothing - it must be included in a calculated field or a filter to affect the viz and finally they are static and global - i.e. the don't change until the user changes them and they are constant across the entire book
Filter filter data out of the vis - so if the plan is to projet the future based on the past you need the full range of data - sounds like filters are not the way to go
set up your 2 parameters - one on date that includes the full range of data (actual and projection) in the data and the second an integer theat includes the years (4 or 5 from you post) and use date functions like dateadd to do do your calculations based on date
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