What is known about the phase of matter at very high temperature and enough pressure?
There is a point in which it is too hot for a molecule to remain bonded and will break apart, no matter how much pressure is applied. Now with more pressure the electrons and nuclei will start free flowing in what is known as a plasma. And add a little more heat and fission happens.

So basically the reason they don't show you any more of the curve because your molecule under enough pressure and heat is no longer a molecule.