My interpretation of your requirement is that you are looking for two different views on your data. One view shows the volume, and the other shows actual vs forecast sales. What I don't understand is why you would want to force theses two things in to the same chart? I think you are over complicating this, my advise keep things simple and manageable.
My main suggestion is that you think of the workflow you want to create in the Qlik Sense app. Essentially, don't force the business user into a guided reporting model, where you dictate what the user should look at. Allow more flexibility around the discovery of your data.
There are two very simple options, which might work in this case.
Create two chart in the same sheet. One shows volume, and one shows sales. This allows your user two look at both things at the same time. If you struggle with fitting the charts into the same sheet, take a look at April 2018 release and the new option to resize layout grid.
Create different sheets. User can navigate to for example a sales sheet and look at more details related to actual vs forecast sales. By leading the app with a overview dashboard, and providing more details through additional sheets you give the user flexibility to navigate the data in many ways.
Regarding your attempted solution. My general advise is that you avoid using calculated dimensions, as this will easily lead you to unexpected results, or in worst case misleading results. Commonly when you want a selective filtering the expression can have set expressions or if statements to control the aggregated data set.
If you are not familiar with synthetic dimensions, I would advise on not using Valuelist() in you charts. This function will not provide you the pairing of values that you expect.
You are referring to two charts. Both have very simple measures, I am not sure why you can not set it up yourself.
You example has a synthetic complexity that does n t make any sense at all to me. Dimensions and measures essentially work the same way in QlikView and Qlik Sense, so if you have something that works in QlikView just apply the same in Qlik Sense?