Describe angular data help

I apologise for not presenting any code, Im not sure how to start..

I want a way to characterise the following plot. The aim is to have some number that describes how close the data is to the 45 degree line. AS i change my experimental conditions, these arms change. I thought about either an angle or sum of distances from the line??, but as their are two arms (although plotted as one data set) - I've no idea how to approach this. Any pointers would be greatly appreciated