Page:Graphic methods for presenting facts (1914).djvu/67

 available in many different colors. Some grades of the drawing ink are water-proof after drying. On elaborate charts the water-proof quality should always be used to make certain that a few rain drops, or handling with moist hands, will not ruin the finished work.

American Review of Reviews

Fig. 52. The Increasing Number of Students in the Co-operative Course of the Engineering College of the University of Cincinnati

This illustration was originally used as a companion piece to the chart of Fig. 51. For a popular exhibit the use of vertical bars brings out information quite clearly. Though curves (such as are shown in later chapters) are superior to vertical bars, it is unfortunately true that most people do not know how to read even the simplest curves correctly.

Note the lettering at the upper portion of the chart. Lettering of this kind may be had by using separate gummed letters such as may be purchased ready for use

In the discussion of Fig. 51 and Fig. 52 it was mentioned that a line could be drawn through the tops of the vertical bars to give a curve. If the curve were actually drawn, the bars themselves would be omitted. In Fig. 53, instead of using bars, we have lines which may be considered as curves for each of the several items compared. In this case it would be impossible to use bars for each of the items shown because the bars would cover each other. The bars are entirely omitted and lines are simply drawn from the 100 per cent point in 1897 to the various points for different items in 1907. It is certain that the prices (that of pine lumber, for instance, shown by the upper curve) did not have the uniform rate of increase which the straight line from 1897 to 1907 would indicate. We are considering here, however, the changes over the period as a whole, and we can for simplicity draw a straight line and neglect all the fluctuations of intervening years. The general scheme of Fig. 53 is convenient, as the neglect of detail brings the main information out clearly. Fig. 53 has, unfortunately, been drawn in a misleading manner in that the reader is likely to interpret the curves as if zero were shown at the bottom of the chart. The general rule in charts of this kind is that zero should be shown as the bottom line, or, if not shown at the bottom, that the omission of zero should be clearly indicated. As Fig. 53 is shown on a percentage basis, the 100 per cent line should be clearly indicated by drawing a broad line on the chart for the line opposite the figure 100 in the scale. It would have been better, perhaps, to have plotted the data so that zero would replace the figure 100. On a scale so made, pine lumber would go up 83 per cent, while railroad rates would be shown