We take imagined class 560-600 and 840-880, each with frequency 0. The class marks of above classes are 5 and 75 respectively.

As we can see in the figure the x-axis starts at 560, a break is indicated near the origin to show that the graph is drawn with a scale beginning at 560, not at origin.

So, we plot the points A (580, 0) and B (860, 0). We join A with the midpoint of the top of the first rectangle and B with the mid-point of the last rectangle.

Thus we obtain a complete frequency polygon,