Page:Graphic methods for presenting facts (1914).djvu/91

 of the three is followed in Fig. 68, where the data from Fig. 67 are plotted in the form of a curve. The curve method brings out all the information in less space and in clearer form than does the block system in Fig. 66.

Fig. 67. Values of Annual Exports and Imports of the United States. Figures are Given in Millions of Dollars

This chart is drawn to scale from the data given in Fig. 66, to show the use of horizontal bars for work of this nature

An understanding of how to plot curves and of how to read them should be part of the equipment of every business man, just as it is of every engineer, physician, biologist, and statistician. The general scheme of curve plotting is so simple that instruction in it should be given as part of the work in elementary arithmetic in all public schools. Children over ten years of age can do plotting and can understand simple curves like that of Fig. 68. Curves are usually plotted on co-ordinate paper already ruled in squares, so that the person doing the plotting need not take the time necessary to rule the paper. For reports, for illustrations of magazine articles, or for advertising, it is ordinarily better to rule by hand a small area such as is shown in Fig. 68 so that the spacing of the ruling may suit exactly the data which it is desired to plot.

Fig. 68. Value of Annual Exports and Imports of the United States. Figures are Given in Millions of Dollars

This curve is drawn from the same data as Figures 66 and 67. The curve shows the changes from decade to decade more vividly than do the horizontal bars of Fig. 67

In Fig. 68 we have data available only by decades, from 1830 to 1900 inclusive. Lines are drawn vertically for each decade. The scale need not run above 2,300, for the largest figure to plot is 2,244 millions. A suitable scale is secured by using one line for each 200 millions. After the background ruling has been drawn, the figures at each census are laid off to scale, on the proper vertical lines to represent census years, and a dot is placed on each vertical line at that vertical distance which represents