In thermodynamics, the pressure-temperature graph for an ideal gas kept at constant volume (isochoric process) is a straight line. This is because for an ideal gas, the pressure is proportional to the temperature (as described by the ideal gas law, PV = nRT, where P is the pressure, V is the volume, n is the number of moles, R is the gas constant, and T is the temperature).

It is clear from graph that for all the gases lines of graphs meet at same value.

At $\mathrm{x}$-axis (temperature axis) $\mathrm{P}$ is zero but temperature is negative and it will be equal to 0 K or $$-273^{\circ} \mathrm{C}$$.