Page:Graphic methods for presenting facts (1914).djvu/277

Bureau of Railway Economics, Washington, D. C.

Fig. 204. Monthly Revenue, Expense, and Net Revenue per Mile of Line for Railroads in the United States Having a Yearly Revenue of One Million Dollars or More

This illustration shows one of the difficulties encountered when curves for successive years are plotted on the same co-ordinate ruling. Here the data of January, 1913, are indicated by a dot rather than by a line. The change occurring from December to January is not easily seen. This difficulty can be easily overcome by allowing an extra space for one month as in the following illustrations

The man who reads the curve, however, must ordinarily get the value of any point on the curve by referring to the scale on the left-hand margin of the chart. As most points on curves do not fall exactly on the horizontal co-ordinate lines, the only way in which the actual value at any point can be determined is by careful estimate of the fractional distance between horizontal lines, according to the scale shown on the margin. The resulting value for the point is ordinarily more or less inaccurate, depending upon the scale to which the curve is drawn. Not only is the reader's time taken in estimating the value for any point on the curve, but when he gets his result he is dissatisfied, for he cannot feel that the figure obtained is really accurate. There is a great advantage in showing on a chart the figures from which the curve was plotted. When only one curve is shown on a chart it is very easy to give the figures immediately above each point. The method used to show the figures in Fig. 203 is not, however, satisfactory, as the figures do not fall in a column. The method of indicating figures shown in the illustrations later in this chapter is much superior to the method of Fig. 203, and should be adopted as general practice by anyone preparing curves for record purposes, or for executive use.