Allow Graph By in window functions

JensenATL

I'm trying to create a dynamic Beast Mode that uses a window function to select the first value of a column (based on date) that is part of a function that sums other columns. In excel parlance I want to be able to vlookup start_date and grab the value for the associated column, then add that to a sum of another column that ranges from start_date to end_date . The key is to allow end users to set their own start and end dates and the function updates dynamically.

And example of the proposed Beast Mode:

1 - (SUM(`Adjustment`)/
   (
(first_value(`X_Qantity`) OVER (PARTITION BY `SKU`, `Alias` ORDER BY `Date`)) +
    SUM(`Y_Quantity`)
    )
  )