Note the use of dimension lines in combination with the scales

148 GRAPHIC METHODS

The use of engineering dimension lines in Fig. 133 is of interest, for the dimension hnes add considerably to the clearness of the drawing. In the center of the chart the vertical dimension lines on both sides of the names for each area show distinctly that the chart must be read on the basis of the vertical distance between the two curves on either side of any area. At the right- and left-hand edges of the chart the over-all dimension lines show the reader at once how to read the chart so as to include all the various components entering into any total which may be under consideration. The use of dimension lines should be thoroughly understood by everyone drawing charts or plotting curves, and by everyone having graphic presentations to read. Dimension lines may add much to the clearness of a chart without being in themselves unduly conspicuous.

Chapter IX CUMULATIVE OR MASS CURVES

THE curves tlius far shown have practically all been of a type in which the thing plotted was a value or a rate per week, per month, or per year. The tendency of such curves is to follow a horizontal direction unless affected by conditions which cause seasonal fluctuations or gradual increases or decreases. In this chapter we shall consider curves in which the data plotted concern total output, rather than the rate of output. With cumulative or mass curves, such as are considered here, each point on the curve represents a total output up to the time for which the last point is plotted. The figure for each successive period of time is added to the total already recorded, and the new total point is plotted. Because the figures relating to the last unit of time are always added to the total figures already recorded, curves of this type are called cumulative curves or mass curves.