Is this gas in thermodynamic equilibrium? Please consider the following graph of molar volume as a function of pressure for a fixed volume.



Can we say whether this system under thermodynamic equilibrium or not?

Update Thinking about this some more, the molar density vs pressure graph may be useful too.



Is this gas in thermodynamic equilibrium?
I don't think "equilibrium" is a good word to use in your question.  We have no way of knowing if the gas is at equilibrium from the data you present.  A highly non-ideal gas that was exploding might give the same relationship.  A gas undergoing a rapid unimolecular reaction wouldn't really be at a true chemical equilibrium but could behave as an ideal gas from a pressure / temperature standpoint.

Did you really mean to ask if the gas was ideal?  An ideal gas follows $PV = nRT$, which can be re-written in several ways:


molar density $\frac{n}{V}$ as a function of pressure $P$: $$\frac{n}{V} = \frac{1}{RT}\;P$$, which is the equation of a straight line through the origin (bottom graph).
molar volume $\frac{V}{n}$ as a function of pressure $P$:  $$\frac{V}{n} = RT \frac{1}{P}$$, which is the equation of a hyperbola (top graph).


Your data is consistent with the gas being ideal (and isothermal).