Opinion: non working days shading should always be shown on the control chart

1. When non working days are excluded from the calculation, there is currently no visual feedback on the chart showing where time was taken out.

2. When non working days are included in the calculation, the fact that there is a weekend is interesting but a nice to have, because really it doesn't affect the calculation

Currently 1. doesn't show the non working days and 2. shows the non working days which is the wrong way around. We should simply make the chart always show the non working days, as it is necessary in 1. and useful in 2.

Comparing this to the burndown chart: there we never decide whether the non working days affect calculation, as they always do. The switch there is to include/exclude the non working days in the chart display, which as a result then adapts the x-axis scale. In the control chart however, we always show the full scale, which as a result should also show the non working days shaded.