How to understand Nelson-Obert charts? The compressibility factor is given by: 
$$
z=\frac{P\overline{V}}{RT}
$$
The Nelson-Obert chart is:



This chart brings the following information:


At very low pressure (Pr &lt;&lt; 1), gases behave as an ideal gas regardless of temperature.
At high temperatures (Tr > 2), ideal gas behavior can be assumed with good accuracy.
The deviation of a gas from ideal gas behavior is greatest in the area of the critical point.


Where:

$$
T_R=\frac{T}{T_{(At\ critical\ point)}}\\
P_R=\frac{P}{P_{(At\ critical\ point)}}
$$

What I don't understand is: 


Does this chart describe the ideal behavior of a supercritical fluid or a superheated gas or plasma?


Having in mind any phase diagram:
It describes the entire region to the right of the vertical dashed line through the critical point.