A graph of gas pressure versus the number of particles in a container is a straight line. Which other relationship will have a similar graph?

From the Law of Boyle for gases you know that the pressure and the volume of gases are inversely related. That, means that you can state PV = constant. And that implies that if P has a linear relation with the number of particles (which is what the straight line means), then V has the same kind of relation. So, the relationship of volume and number of partilces will have a similar graph.