Summing Forecast with Historical Data

[gitguto]

Hey Folks,

I have a column "Version" that contains weekly forecasts of costs. Every week a new W# is uploaded into my database. It forecasts the costs of the remaining months of the year (W1 - W4 will forecast Jan - Dec; W5 - W8 will forecast Feb-Dec; W9-W12 will forecast Mar-Dec, and so on).

I need to replace the empty values of the W# with the Historical costs of that month. (See example below)

I need this in order to see the weekly evolution of my planning (image below), the bar length formula is Sum({<Date = {"*2021*"}>} [Cost]). The problem is that from W5 forth the bar will become shorter and shorter because there will be no values in Jan from W5 on, no values in Feb from W9 on, no values in March from W13 on, etc.

I tried creating a master dimension but I'm stucked in the formula. 

Any help will be appreciated, thank you for your attention.