Page:Graphic methods for presenting facts (1914).djvu/105

 ordinarily the independent variable when it enters into curve plotting, nevertheless there may be occasions when time is the dependent variable, and charts should be plotted accordingly. It is important that the person drawing a chart should in each case distinguish between the independent variable and the dependent variable, for this distinction affects the whole arrangement of the chart.

It should be a strict rule for all kinds of curve plotting that the horizontal scale must be used for the independent variable and the vertical scale for the dependent variable. When the curves are plotted by this rule the reader can instantly select a set of conditions from the horizontal scale and read the information from the vertical scale. If there were no rule relating to the arrangement of scales for the independent and dependent variables, the reader would never be able to tell whether he should approach a chart from the vertical scale and read the information from the horizontal scale, or the reverse. If charts are always plotted with the independent variable as the horizontal scale, there need be no question in the reader's mind as to how he should interpret the chart. The rule for scale arrangement is not always followed, and a few examples are shown here to indicate the difficulty of interpretation which the reader may have just because a rather simple principle of curve plotting has been neglected.

Metropolitan Sewage Commission of New York, 1912

Fig. 81. Number of Bacteria per Cubic Centimeter of Hudson River Water at New York at Different Depths below the Surface

In Fig. 81 the depth of the water has been plotted downward from the top of the chart so that the reader may get the impression of measurements taken at different distances below the surface of the water. In making the tests which are represented in Fig. 81, different depths below the surfaces were selected and the bacteria determined from the water samples taken at these depths. The depth is here the independent variable, and bacteria per cubic centimeter the dependent variable. The decision as to which is the independent variable and which is the dependent variable rests entirely on how the problem is approached. Numerous samples could have been taken at different depths, and then a curve plotted to determine the depth at which certain numbers of bacteria per cubic centimeter were found. In