We know $V\propto I$, so $V-I$ curve for a ohmic devices is a straight line passing through origin. Due to some errors/carelessness during experiments, all points may not come on the same line. In this situation, we draw the most appropriate curve.

From given $V-I$ curve,
$\Rightarrow \mathrm{R}=\frac{2.5}{2.5}=1\mathrm{ }\mathrm{\Omega }$