Page:Graphic methods for presenting facts (1914).djvu/153

 rule will make them sufficiently clear. A person who can plot a chart to the ordinary scale should have no difficulty in using the logarithmic scale.

No matter what the location on the chart, if the logarithmic spacing is used on the vertical scale, for curves, the angle of the upward or downward inclination is the same for all curves affected by the same percentage of change. Curves having an increase equaling the distance from 100 to 200, 200 to 400, 300 to 600 (or the distance between any number on the scale and double that number) have an increase of 100 per cent and show the same slope. It will be noticed, for instance, from any paper ruled logarithmically, or from Fig. 123, that the distance on the logarithmic scale from 10 to 20 is the same as from 200 to 400.

In Fig. 122, we have curves plotted for comparative study in the manner most convenient when ordinary arithmetically ruled cross-section paper is used. Some of these curves represent large quantities, so that they are on the upper portion of the chart, while others represent comparatively small quantities and fall near the bottom of the chart.

Just because the curves in the upper portion of the chart represent numerically larger quantities, they have much more vertical movement up and down on the face of the chart than those curves in the lower portion of the chart which may have an even greater amount of percentage fluctuation. This wide difference in the amount of vertical movement on a page is one unfortunate source of confusion to persons who are just beginning to study curve charting.

Fig. 123 is plotted from the same data as Fig. 122, but it is on paper having logarithmic spacing for the vertical scale with the ordinary arithmetical spacing for the horizontal scale. With the logarithmic spacing on the vertical scale the fluctuations in the different curves show in true proportion. Curve F appeared insignificant in Fig. 122 because it happened to fall near the bottom of the chart where percentage fluctuations are not prominently shown. In Fig. 123, however, curve F shows up as having far the greatest percentage changes of any curve on the whole chart. For persons who understand even slightly the principles involved in reading charts plotted on logarithmic paper, Fig. 123 shows up the facts in much more convenient form than Fig. 122. To make comparison most convenient, the two figures are placed on facing pages, 134 and 135.