Page:Graphic methods for presenting facts (1914).djvu/218

 H. F. Thomson, Massachusetts Institute of Technology, Vehicle Research

Fig. 162. Cost of Gasoline in Cents per Truck-Mile for Different Sizes of Motor Trucks

Since motor trucks are rated by the manufacturers in multiples of 1,000 pounds, the dots representing the records of different trucks naturally fall on the lines spaced by 1,000-pound intervals on the chart. The solid line is drawn through points at the "center of gravity" of all dots for any vertical line on the chart and represents the average condition as reported by truck operators. The dotted line is drawn through points on the chart for data furnished by the manufacturer as his estimate of good practice

A curve of this general type, proving a close relationship between two variables, may be called a correlation curve

Diagrams made on the scheme of Fig. 161 are of very great assistance in studying such matters as the distribution of wealth for any country. The subject of wealth distribution is so complex that unless data are expressed graphically there would be very serious difficulty in getting a true understanding and appreciation of the different factors involved. In Fig. 161 we have the curves for two widely separated years plotted side by side, so that we may tell from the general shape of the curves whether the distribution of wealth is approaching uniformity of tending in the direction of great concentration in the hands of a few people. The more nearly the curve approaches a straight line the more nearly wealth is distributed uniformly among all the