The distance a baseball travels when a player hits the ball varies directly with the speed with which the player swings the bat. The graph below shows the distances a baseball travels (in feet) and the speed of the swing (in miles per hour). A graph titled Baseball Speed has speed of swing (in miles per hour) on the x-axis, and distance (in feet) on the y-axis. A line goes through points (0, 0), (40, 200), (60, 300).

Based on the graph, which relationship exists between the speed of the swing and the distance the ball travels?