Page:Graphic methods for presenting facts (1914).djvu/115

 by referring to the figures near the upper portion of the chart. Note that in 1908, 21 per cent of the services were metered, while in 1911, 45 per cent were metered. The proportion of the services metered in 1912 was not known at the time the chart was made and hence is not recorded.

Courtesy of Hazen & Whipple, New York Fig. 90. Chart Showing by Months the Average Total Daily Water Consumption in Boston, and by Months the Average Daily per Capita Water Consumption. Also the Yearly Average of Daily Consumption Stated in Total and per Capita

In this illustration the curves may be read from either of two different sets of co-ordinate rulings. Using the horizontal ruled lines, we may read from the curves the average total consumption per day. By reading from the slanting lines, the same curves may be interpreted as the average consumption per capita per day. The scheme of using two sets of co-ordinate rulings is a valuable one. The scale for "million gallons per day" should, however, have been shown only at the left, with the slanting line scale for "gallons per capita" placed in the right-hand margin for the sake of clearness

A little study of Fig. 90 will show that there is a very striking similarity in the shape of the waves for different calendar years. Water consumption is high in the winter and again high in the summer months, with the lowest point each year usually found in November. The exact amount of resemblance of these waves to each other could be determined in an interesting manner if a separate curve were plotted for each year so that all the curves would be shown one above the other in the manner seen in Fig. 103.