The purpose of trigonometric axes in plot of electron clusters, ATLAS

[kimi7335]

TL;DR

Why use trigonometric axes for the plot?

I have been doing some reading on electron reconstruction from proton-proton collisions at CERN. In some of the papers I have read, plots such as the one In the figure are included. What I would like to know is why they have chosen to plot the x and y axes as cos(phi) * tan(theta) and sin(phi) * tan(theta)?

 

[mfb]

Angles can be compared across experiments, physical dimensions cannot. In addition the calorimeter has some non-zero depth, so just plotting x and y would smear out clusters that point away from the interaction point. You could project everything onto a fixed plane, but that's just extra effort for a less useful result.