One mole of an ideal monoatomic gas undergo a thermodynamic cyclic process ABCDA as shown in internal energy (U) and density ($\rho$) graph.

The corresponding temperature (T) versus pressure (p) graph is

(a) Here, AB and CD are isothermal and BC and DA are isochoric . Now it is easier to draw T-p graph for the given situation. AB and CD isothermal process, in AB volume increases and in CD volume decreases, BC and DA that constant volume process So. p –T graph straight line passes through origin.