I found your post very useful. I have requirement to do the same thing in Qlik Sense.
However I already have the Min Max Actual and Forecast values coming in as fields from database. My challenge is to shade the area between Min and Max, something like the image attached. Could you please give some suggestions as to how it can be achieved using Vizlib Line Chart extension. As I understand we do not need to use the advanced section forecasting for this since I already have the forecasted values.