Generic Inquiry question - Adding two Item Quantities together

I a working on a Dashboard and one of the items the user want to see is a comparison of items sold, in a date range. The user wants to see the results in a doughnut chart. 

There are three items that will figure into this chart, the quantity sold of two of the items needs to be added together and displayed as one of the elements in the doughnut chart.

As an example lets same the three items are Light Blue widget, Dark Blue widget, and Red Widget.

We want to combine the quantity sold of the Light Blue and Dark Blue together and show that as one element of the doughnut chart, and the Red Widget total sold would be the second element of the doughnut chart.

My question is, how do I total the two Blue widgets together in the Generic Inquiry