Calculate Difference between Legends in Clustered Column Chart

Help Needed!!

I have a clustered chart, showing columns based on selection from a slicer. 

I want another chart showing the difference between the legends from the same column group. (If there are more than two selections selected for the legends, it should show the largest difference, but this is optional as the selections would usually be two) 

Is there a way to make this measure? 

Thank you