The pressure of a gas is directly proportional to its temperature when the volume is constant. The ratio of pressure to temperature is constant when the volume is constant. A constant volume process is said to be isochoric.
From the equation of state for an ideal gas PV=nRT, we get the following.
1. P=nRTV, i.e, P∝T(V=constant)
At P=P0, and V=V0, we get T=P0V0nR
And at P=4P0, and V=V0, we get T=4P0V0nR
The graph is a straight line (passing through the origin, when produced) shown in figure(i)
2. Since temperature increases from P0V0nR to 4P0V0nR
Volume remains constant; so, the graph of volume-temperature will be as shown in the figure(ii).