Revenue is quantity multiplied by the price, this can be plotted in a graph (see left). Multiple periods can be plotted on top of each other.  As with a WAPI calculation, this should be calculated on a customer / article level and aggregated.

This quickly shows where changes in either price of quantity have an effect on volume. The ? area top right will be discussed below how to handle this.

There are two elements to the designated rectangle. Firstly, it can be assigned to either price, volume or  split in two trapezoids. 

Secondly, there is no filled retangle when there is growth in one axis and shrinkage in the other axis. When using a single formula for all situations to calculate the surface, the rectangle will be either calculated twice, or not at all, resulting in what most people call a mix effect.