Page:Graphic methods for presenting facts (1914).djvu/372

 Fig. 246 gives another example where neglecting to show the zero line may cause an entirely erroneous impression regarding the facts which the chart is intended to bring out. The failure to show the zero line at the bottom of a chart is so common a fault, found in nearly all publications, that some typical examples are shown here in the hope that a bad practice may be somewhat reduced.

Fig. 247 gives the data of Fig. 246 redrawn so that the zero line is shown at the bottom of the chart. It is believed that this illustration will prove conclusively how great an error may be made if charts are read hastily on the assumption that the bottom line of the chart is the zero line. Since some persons are almost sure to read a chart from the bottom line, it seems desirable that all charts should be so made that the reader may interpret from the bottom line as a zero line, or else receive positive warning that he should not do so.

Data of Elmer ouse

Fig. 248. Comparison of Death Rates in the United States, Showing Reduction in Death Rate for Tuberculosis and Increase in Death Rate for Degenerative Diseases

This illustration is identical with Fig. 246 except that here a wavy line is used at the base showing that the bottom of the chart is not at the zero of the vertical scale. It is always desirable to have the bottom line at zero. If that is not possible the wavy line should be used as a warning to the reader

Though there is no necessity for showing Fig. 246 without having a zero line at the bottom of the chart, Fig. 248 is presented here to prove how easy it is to make a wavy line at the bottom of a chart if there should be any real reason why the chart cannot be made so as to include the zero line of the vertical scale. Fig. 248 is exactly the same as Fig. 246 except that the wavy line is used instead of the straight line at the bottom.

The beginner in curve plotting and in curve reading is apt to be somewhat puzzled by the different effects which may be obtained by changing the ratio between the vertical scale and the horizontal scale. It is difficult to give any general rules which would assist in overcoming the beginner's confusion. Ordinarily the best way to get facility in making the proper choice of vertical and horizontal scales for plot