Page:Graphic methods for presenting facts (1914).djvu/159

 To show that the chart is absolutely limited to the height representing 100 per cent, we use a broad line for the zero line and another broad line at the top for the 100 per cent line. Instead of showing the percentages at different decades by the method of shaded bars used in Fig. 125, the vertical lines representing decades are first marked with points dividing the lines into component parts, then the points on the various lines are joined to give a curve. The area underneath the curve is shaded in this illustration simply to give a greater contrast between the two portions of the chart. Charts of this kind made with shaded or colored areas are understood by a surprisingly large number of people who ordinarily would not understand a chart made by using curve lines without the shaded or colored areas.

Fig. 126. Percentage of United States Foreign Trade Carried in American Vessels and in Foreign Vessels by Decades, 1820 to 1900

This type of chart requires a more highly educated reader than the type of chart shown in Fig. 125, but it gains by making the information stand out more clearly than possible with a series of bars

The double scale at the right of Fig. 126 is worth noting. The percentage for the United States can be read for any decade. The percentage for foreign vessels can also be read for any decade by using the reversed scale, in which zero is placed at the top and 100 per cent at the bottom. Though a double scale is scarcely necessary on a chart as simple as Fig. 126, it is frequently desirable to have a double scale.

Another very striking wall chart is shown in Fig. 127. Here as in Fig. 125 the chart was framed, but the frame shows in the photograph only as a black border. In making up this chart co-ordinate