Assuming you have one dimension (Sector) and two expressions (first is paper orders and second is computer orders) in a stacked bar chart, you can use the Y-Value sort checkbox instead of the Expression. This will render the chart showing first that sector with highest value for the first expression (paper orders).
According to this, create a new expression:
Sum(paper_orders + computer_orders)
Uncheck the "Bar" tick in the expression (so there will be no new bar with this value), and check "Values on Data Points". Now promote this expression to the first place.
Now you get it.
Hope that helps.