The vertical axis is labelled incorrectly. It should read $$\frac{PV}{RT}$$where V is the molar volume. This is called the compressibility factor Z. According to the Principle of Corresponding States, Z is a function of the reduced pressure ##P/P_C##, the reduced temperature ##T/T_C##, and the ascentric factor ##\omega##, where the subscript C indicates the value at the critical point and where ##\omega## varies with the specific gas. Graphs of Z as a function of reduced temperature, reduced pressure, and ascentric factor can be found in most thermo books.phantomvommand said: