Robert recorded the number of miles he had biked every hour of a 4-hour bike trip. He graphed the results on the grid below.
The graph titled Robert's bike trip shows distance in miles on the vertical axis from 0 to 80 and time in hours from 0 to 4 on the horizontal axis. The points (0, 0), (1, 18), (2, 35), (3, 51) are joined to form a line.
Which statement BEST explains whether or not Robert’s graph represents a proportional relationship?