Page:Graphic methods for presenting facts (1914).djvu/96

 between the lines of the circles, rather than the whole area included inside of each circle line.

Fig. 73 gives the data of Fig. 72 in curve form. The heavy solid line curve shows the changes from decade to decade as they could never be interpreted from either the actual figures or the circles of Fig. 72. The tremendous increase in the world's commerce between 1900 and 1910 is of very great interest, showing the effect which better means of communication have brought about as a result of the splendid increase in scientific and engineering knowledge.

The Philadelphia Commercial Museum

Fig. 72. Annual Commerce of the World, Imports and Exports Combined. Shown at Ten-year Periods, 1850-1910

These circles, drawn on an area basis, are even more difficult to interpret than the circles of Fig. 38. The eye is likely to see the rings rather than the whole areas of the circles. Compare Fig. 73

The Philadelphia Commercial Museum

Fig. 73. Annual Commerce of the World, Imports and Exports Combined. Shown at Ten-year Periods, 1850-1910

The solid line is plotted according to the figures given in Fig. 72. The dotted line shows the erroneous impression which would be obtained by the reader if he should interpret Fig. 72 by the diameters instead of by the areas of the circles

If one makes comparisons between the circles of Fig. 72 on a diameter basis rather than on the area basis to which the circles were drawn, one gets an interpretation like that indicated by the dotted line in Fig. 73. By comparing the dotted line with the solid curve the reader may see the extent of the error which arises if circles are compared on a diameter basis after being drawn on an area basis.

Fig. 74 gives a good idea of the utility of the curve method for showing concisely a large quantity of data. If the figures for the price of cement had been expressed in dollars and shown in a long numerical column, there would be very few readers who would take the