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Fig. 80. Yearly Average of Revenue Tons per Train Mile on the Pittsburgh and Lake Erie Railroad. The Slanting Line Shows a Progressive Average

Here we have the data of Fig. 79 plotted in a curve which can be interpreted easily and accurately This chart may be considered a model of good practice in curve plotting. All of the work, including the lettering, has been done by hand, thus insuring better results than can usually be obtained from printing

The following features of Fig. 80 are pointed out for the benefit of anyone who may have curves to plot:

1. The zero line is a much broader line than the co-ordinate lines.

2. Heavy lines are not used at the right- and left-hand edges, since the chart does not start or end at the beginning or end of time.

3. All lettering is so made that it can be read horizontally or from the right-hand edge of the sheet.

4. Years are given with four figures for every tenth year ending in zero. Other years are indicated with two figures so that they may be more quickly read.

CURVE PLOTTING 83

5. All letters and figures on this chart were made by hand, showing the perfection which may be attained by practice in lettering.