The graph of  $\ln \left(\frac{R}{R_0}\right)$ versus  $\ln A$ is (is the radius or a nucleus and A is its mass number)

we know,  
 $R=R_0{A}^{1/3}\Rightarrow \frac{R}{R_0}=A^{1/3}\Rightarrow \mathcal{l}n\left(\frac{R}{R_0}\right)=\mathcal{l}n\left(A^{1/3}\right)\Rightarrow \mathcal{l}n\left(\frac{R}{R_0}\right)=\frac{1}{3}\mathcal{l}n\left(A\right)$

This equation represents a straight line