The blue line shows the compression factor for hydrogen gas. The default values for the red line are for ethane. The black line is for an ideal gas (Z = 1). Use the sliders to change the van der Waals parameters and the temperature.
A table of van der Waals parameters is shown below.

Gas a b
Ammonia 4.169 0.0371
Argon 1.337 0.0320
Chlorine 6.260 0.0542
Ethane 5.507 0.0651
Hydrogen 0.242 0.0265
Oxygen 1.364 0.0319
Water 5.464 0.0305
Ideal Gas 0 0