But "depth" is literally the down direction, so I think it's also acceptable to do this y/x thing. I've seen this choice in other places too,

https://en.wikipedia.org/wiki/U.S._Standard_Atmosphere#/medi...

https://en.wikipedia.org/wiki/Climate_of_Titan#/media/File:T...

https://en.wikipedia.org/wiki/Thermocline#/media/File:Thermo...