Page:Graphic methods for presenting facts (1914).djvu/173

 and a half by eleven inches (a standard letter-sheet size), and has co-ordinate ruling printed in green ink with wide margins on all four edges to allow space for lettering, scales, etc. Note in Fig. 134 that the green ink of the co-ordinate lines shows much lighter in color than the black drawing ink used for the scales and the curves. Though the co-ordinate lines are distinct enough for ease in reading they are not nearly so conspicuous as they would be if a line cut had been used instead of a half-tone. In making a line cut, the green lines of the paper must, of course, be printed as black and the color value of the green lines is entirely lost. The finished line cut shows only the relative widths of lines, not relative colors. For many illustrations of curves, conspicuous co-ordinate lines are not desired. In such cases it is better to use a half-tone, as has been done in Fig. 134, than to use a line cut.

In Fig. 135 we have curves for two successive fiscal years plotted so that they may be compared easily. The double-line curves and the dotted-line curves are plotted to show the rate of shipments, while the heavy-line curves are plotted on a cumulative basis and give the total number of carloads shipped since the beginning of each fiscal year. Ordinarily it is not desirable to put on one sheet of paper curves giving both rate of output and cumulative output, as there is danger of confusing in the reader's mind two different types of curves. This illustration is included simply to show the possibility of comparing two cumulative curves for succeeding years by plotting both cumulative curves on the same sheet.

It must be remembered that cumulative curves always refer to some definite length of time and that they must always begin at the beginning of the period for which the summation is made. Cumulative curves do not extend outward indefinitely, but start over again at zero with each succeeding period of time. Thus, cumulative curves plotted by months or weeks on a long sheet of paper, for a series of years, would be seen in the shape of saw teeth, with the highest point at the end of each fiscal year and then a drop to zero again at the beginning of the next fiscal year. The saw-tooth shape makes it feasible, and sometimes desirable for easy comparison, to plot cumulative curves for several successive periods of time in the same space, as the two cumulative curves in Fig. 135 are plotted.

The progressive average shown by means of the dotted lines in Fig. 135 is obtained by averaging each month the values for all the points