Variation of internal energy with density of 1 mole of monoatomic gas is depicted in given figure. Corresponding variation of pressure with volume can be depicted as (assume the curve is rectangular hyperbola) : 

Since : U \(\rho\) = constant

\(\frac{P}{\rho} = \frac{RT}{M}\)

P = constant since \(\rho\) is increasing, therefore V is decreasing