Graphic Methods for Presenting Facts
Lines are drawn vertically for each decade. The scale need not run above 2,300, for the largest figure to plot is 2,244 millions. A suitable scale is secured by using one line for each 200 millions. After the background ruling has been drawn, the figures at each census are laid off to scale, on the proper vertical lines to represent census years, and a dot is placed on each vertical line at that vertical distance which represents
MlLUONS OF --
1S04
/
1 800
/
leoo
/
/
tooo
/
/
/
>
y
\r
^
1630 40 '50 '60 "70 '80 '90 1300
Fig. 68. Value of Annual Exports and Imports of the United States. Figures are Given in Millions of Dollars
This curve is drawn from the same data as Figures 66 and 67. The curve shows the changes from decade to decade more vividly than do the horizontal bars of Fig. 67
72 GRAPHIC METHODS
the data according to the scale chosen. Thus the figure 222 for the year 1840 would be indicated as a dot on the vertical line for 1840 slightly above the place where the horizontal scale line for 200 crosses the chart. After the dots for decade years have been placed on the vertical lines, the dots are joined with a heavy line and a curve appears.
The curve of Fig. 68 shows the changes from decade to decade much better than the bars of Fig. 67. It can be seen at once from the curve that the greatest gain in any decade was between 1870 and 1880 when the increase was three and a half divisions on the vertical scale. The bars in Fig. 67 have no horizontal scale to measure by and the comparison between census years is accordingly more difficult.