Page:Graphic methods for presenting facts (1914).djvu/106

 such a case, bacteria per cubic centimeter would be the independent variable and depth would be the dependent variable. This sort of problem may be attacked from either one standpoint or the other, and it is just a question of convenience as to which method is used and which variable is made the independent variable. Though the problem can be stated in such manner that either one variable or the other can be made the independent variable, after the statement has been made the chart should be consistently drawn so that the independent variable will be used as the horizontal scale and the dependent variable as the vertical scale.

As Fig. 81 is shown it is necessary for the person interpreting the chart to select from the vertical scale some number of feet below the surface and then read the number of bacteria per cubic centimeter by the horizontal distance to the right. It is only after some little puzzling that the reader will notice that the scales for the variables have been reversed and that the chart has been practically turned on its side. How this chart would appear if the horizontal scale were used for the independent variable may be judged by turning the book and looking at Fig. 81 from the left-hand side of the page. Though it is easy to see why the person making Fig. 81 happened to arrange the chart in the manner shown with the variables reversed, the gain due to showing depth below the surface in the vertical direction does not make up for the possibility of misinterpretation which results because of the neglect to follow standard practice.

In Fig. 82 we again have depth plotted downward from the top of the chart. As we wish to determine the velocity of the stream at different depths of water, depth is the independent variable and velocity is the dependent variable. The arrangement of Fig. 82 is not as objectionable as Fig. 81, for the upper half of the illustration shows quite clearly in pictorial form that the subject under consideration is a stream having a channel shaped as shown, with widths and depths as indicated by the two scales. In the bottom portion of the diagram the scale of depths downward relates very definitely to the upper portion of the illustration so that the reader cannot easily go astray. Notice that the curves for the velocity of the water are each plotted on a separate vertical line which serves as zero line. The curves for velocity begin at various points depending upon the thickness of the ice, as will be seen from the upper portion of the chart. There is, of course, no velocity in that portion of the stream which