Page:Graphic methods for presenting facts (1914).djvu/145

 This is especially true where curves are plotted showing the fluctuations in the market prices of stocks or bonds. In such cases, it is desirable to plot two curves, one showing the minimum prices and the other showing the maximum prices. When the two curves lie quite closely together, it frequently assists in the clearness of the chart if the co-ordinate lines are erased between the two adjacent curves. The erasing of the co-ordinate lines makes the curves stand out much more distinctly than they otherwise would.

The Engineering Magazine

Fig. 115. Typical Curves Showing the Twenty-Four Hour Demand for Electricity during the Summer Months in Various Types of Buildings in New York City

Curve D shows the fluctuations in the load on a power house supplying all of the buildings represented by curves A, B, C, E and F. By combining loads of different types, the power plant can be kept more continuously busy than otherwise possible. After the other curves are all plotted, curve D can easily be located by stepping off with draftsmen's dividers on each vertical line the heights of all the different curves at that particular line

Charts on the general type of Fig. 116 are valuable to give a vivid comparison. A chart of this kind would be especially striking if used in advertising, or in a report where concentration upon only one general idea was needed, without a great amount of specific detail. Though Fig. 116 shows that telephone rates have had a constantly downward