In the volume-temperature graph for an ideal gas, the relation between the pressure and temperature for an ideal gas is given by the equation of state:
\[
P \propto T
\]
Since the graph shows two different pressures at different temperatures, the relation between \( P_1 \) and \( P_2 \) is found to be:
\[
P_1 = \frac{P_2}{2}
\]
Thus, the correct relation is \( P_1 = \frac{P_2}{2} \).