Page:Graphic methods for presenting facts (1914).djvu/144

 Fig. 115 gives a good example of a total curve made by adding the figures for different curves. Instead of using addition to get a set of figures from which a total curve may be plotted, it is easier in most cases to get the location of the total curve by the graphic method. All that is necessary is to lay off, with draftsman's dividers, successively on each vertical line, the height above zero at which each of the different curves intersects that vertical line. The totaling curve is drawn through the points thus found. When there are not too many curves, this method answers admirably. It sometimes happens that actual observations for the data of different curves are not simultaneously taken and, for this reason, it may be impossible to add the actual numerical data so as to plot a total curve. In such cases, the graphic method of stepping off the height for the total curve is practically the only one available. After each of the separate curves has been plotted from such data as may exist, it is a very simple matter by the graphic method to locate the total curve from the separate curves. A sufficient number of vertical lines are used to bring the points on the total curve close enough together to represent fairly the data of the separate curves which are totaled.

Adapted from Pennsylvania Farmer

Fig. 114. Chart to Show the Dependence of Corn Yield upon the Quantity of Rainfall during the Month of July. The Yield of Corn is Given in Bushels per Acre

This chart is misleading. The close similarity of the two curves has been obtained by plotting one curve on a larger scale than that used for the other curve. The rainfall curve has been plotted with the bottom of the chart as zero. The corn-yield curve is, however, drawn with the scale starting at twelve.

In plotting curves relating to prices, it frequently happens that there is a necessity for showing in the chart both the upper and lower limits to which the prices may fluctuate in any given period of time.