Page:Graphic methods for presenting facts (1914).djvu/101

 above. The dotted line, of course, coincides with the solid line at the first point where there is only one point to consider in the average. Figures for the dotted line are obtained by averaging the figures for the first two years, then the first three years, then the first four years, etc., until the last point on the dotted line represents an average for all the points on the solid line.

Pittsburgh and Lake Erie Railroad

Fig. 79. Yearly Average of Revenue Tons per Train Mile on the Pittsburgh and Lake Erie Railroad. The Slanting Line Shows a Progressive Average

If this illustration is observed with artificial light, an interesting optical illusion may be noticed in that the slanting line appears blue in color

The use of horizontal bars here gives a chart less easy to interpret than the curve shown in Fig. 80 for these same data

Fig. 80 is worthy of attention as a model of good practice which may be studied carefully by anyone just beginning to plot curves.