Every gas obeys the same universal reduced Van der Waals equation once pressure, volume and temperature are scaled by the critical point (Pc, Vc, Tc):
Pr = 8Tr / (3Vr − 1) − 3 / Vr²
Below Tr=1 the raw curve loops back on itself (unphysical). The Maxwell equal-area rule finds the flat vapour-pressure plateau Psat for which the exact antiderivative ∫PrdVr agrees on both sides — found here by bisection, not by eyeballing an area.
The molecular panel is a genuine kinetic-theory illustration: particle speed scales with √Tr, and the box width tracks the piston volume Vr directly, so compressing the piston visibly raises particle density.