When a body is movig with zero acceleration position-time graph is straight line.
When a body projected vertically upward it moves with uniform acceleration. In the given graph, at tm body reaches to maximum height and from that point body comes down, it is accelerated downwards under the influence of a qravity pull. So the motion is unifromly accelerated. Hence, the displacement time graph of a vertically upward projected body appears as a parabola.