Empty values changed to zero in cross table should be used in over calculation

In version 7.7 it is possible to change empty "---" values to 0 or some other value. When using over statement in cross table this values are not taken into account in calculation. For example, you would like to calculate the difference in sales by customer. Customers having sales in year 2016 (second column) and not having any sales record in year 2015 (first column) will have --- in year 2015. Column axis is made by Year(Sales date). Even if "---" are replaced by 0, Spotfire cannot calculate the difference between year 2016 and non existing values for year 2015 even if they appear now as 0. This makes impossible to sort biggest winners/losers on the market appropriately.