Page:Graphic methods for presenting facts (1914).djvu/162

 plotted as a separate curve measured from zero as a base line. It can be seen at once in Fig. 129 that the component for "Conducting Transportation" increased rapidly until 1895, ran along fairly uniformly to 1900, then slightly decreased, then increased again. By having each curve plotted separately with the points measured from a zero base line, the eye can judge instantly and accurately the changes which have occurred over a period of years in any component which enters into the total. In an illustration like Fig. 129 it should be shown in the title, or preferably on the chart itself, that the sum of the heights of all the curves given on the chart is constantly 100 per cent as indicated by the broad line at the top of the chart. The reader will then know that if any one curve on the chart goes up, some other curve or curves must come down in order that the 100 per cent line may remain straight and horizontal.

Adapted from Railway Age Gazette

Fig. 128. Percentage Distribution of the Expenses of Operating the Railroads of the United States

Here a number of different factors enter into the total 100 per cent. Since the strips representing different expenses vary at both top and bottom, it is not easy to see from year to year how much any strip may be increasing or decreasing. Compare with Fig. 129

Fig. 130 is an interesting application of the method of using areas to show components with 100 per cent shown as a straight line at the top of the combined area. As in this case a large part of the construction work was finished, the actual number of accidents in the construction department dropped to almost nothing, and, because the shaded area for construction grew less, it was necessary that the other areas should widen out if the 100 per cent line at the top were to remain a straight line. Here the weak point in the method of charting is the same as that indicated for Fig. 128. The person observing the chart has no way of telling whether the factors included in the 100 per cent have grown less or grown greater, and whether