This example is an exceptional case and it is named here only to show that, although time is

CURVE PLOTTING 85

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 varial)le 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 so' or/- piEfj-/!- few examples are shown here to indicate the

(JImple" 3soTo 3Si) difficulty of interpretation which the reader

„ „ ^ , , may have iust because a rather simple prin-