Is this a phase change in a gas under Thermodynamic Equilibrium? The above graph shows the molar density (i.e. n/V from PV=nRT) of a gas plotted against pressure. There are two regions with good fit to a linear relationship which I have coloured green and blue (though these are all from a single set of measurements. The visual distinction is just to highlight where the linear fit applies)

My rusty physical chemistry knowledge suggests that the regions where there is a linear fit are in Thermodynamic Equilibrium as the Ideal Gas law would appear to be holding. Given that we have two linear regions I would also conclude that there is a phase change and that the whole is actually in Thermodynamic Equilibrium when we account for the phase change (i.e. n is different between the two regions) 

Is this naïve analysis based on a foggy recollection of Phys Chem lectures from 20 years ago correct or is there another explanation for this data (and if so what is the explanation)

Update

To clarify on the hypothesized phase change. The suggestion is that this phase change is a X + X &lt;=&gt; X2 phase change where X and X2 are both gaseous but the X2 form is only thermodynamically stable at lower temperatures (which correspond to the lower pressures). i.e. not the more common gas-liquid phase change
I don't think it necessarily suggests a phase change.  It just seems to me that, at the higher pressures, we are seeing non-ideal gas behavior setting in.