The statement "Revenue Tons per Train Mile" at the upper left-hand corner is purposely printed diagonally so that it may serve as a heading for each of the two columns of figures, one at the left and the other at the top of the chart. The diagonal arrangement gives a neater effect than can be obtained otherwise.

10. Though figures for the dotted curve could be shown at the top of the chart the dotted line is of only minor interest here. It is accordingly best to avoid the two columns of figures at the top in order that the figures for the main curve may stand out more prominently.

Chapter VI CURVE PLOTTING CONTINUED

THERE are so many different applications of curves and such varied yet convenient methods of plotting curves, that it seems worth while to take up some of these in detail, and point out certain advantages and disadvantages of different curve-plotting schemes.

Practically all curves display relations existing between different sets of data which we may call "variables". One of the variables is used as a standard or measure by which to interpret the facts under consideration, and it may be called the "independent variable". The other variable, which is interpreted from the independent variable, is called the "dependent variable". For example, in a bacteriological examination of a pond at varying depths, distance below the surface would be the independent, and number of bacteria per cubic centimeter the dependent variable. In a seasonal gauging of a stream the dates of observation would be the independent and cubic feet per second of flow the dependent variable. Sometimes we consider more than two variables simultaneously, and we then have two or more independent variables from which to consider a dependent variable.