I am working on automating the creation of a daily FTP curve which was easily enough accomplished below by referencing the 3 yield curves used in construction and stitching together.

My problem occurs when I need to create a monthly curve spanning the entire tenor range. My Excel brain began by inserting the synthetic tenor ranges into the base curve above thinking I could easily perform a linear interpolation for the (null) values using the points above and below each (null) section. However it does not seem that I can perform the interpolation in this manner (you can ignore the FTP Rate column, that would be a simple formula adding Base Rate and Blended TLP so no interpolation needed there).

There is likely a simple solution but given I have not used Alteryx for this in the past it escapes me. Any help would be greatly appreciated. THANKS>