Finally something I can apply my expensive ChemE degree on :')

These diagrams were always the worst to use, far too busy and usually printed on cheap paper making it impossible to distinguish one line from the other.

Basically its a diagram is relating the energy required to change the state of something (usually water) and while in theory is amazing and clean in allowing you to determining enthalpy necessary for a state at a constant pressure.

In practice though can get really messy especially with more variables in play, as shown with the temperature decreasing as the balloon altitude increases.