Beast Mode for Calculating YoY % Variance

Hi there,


I'm trying to create a grouped bar chart (retailers x-axis, sales y-axis, year series) and I'd like to add a line for variance between this year and last. The problem is I'm not sure how to create a Beast Mode to calculate a column for % difference YoY. Essentially, I want:


(`Values` where `Year`='2018')-(`Values` where `year`='2017'))/(`Values` where `year`='2017')


Any help is greatly appreciated!