EDITED VERSION:
I read somewhere that:
During the isothermal isobaric compression of saturated water vapor, there is a region in the PV diagram where we get a straight line parallel to V axis. It is mentioned that at every point of this line, water and steam are in equilibrium.
I need an explanation to the previous line.
You might want to edit your question. Just post a gist to the question in the title, and the complete question in the description.