A curve of uniform slope on any chart of rectangular co-ordinate lines indicates only that there has been a uniform increase or decrease in actual numbers, not

Fig. 69. Average Income of 155 Princeton Graduates of the Class of 1901 for Ten Years After Graduation

Note the effect of the 1907 panic on incomes in 1908

GRAPHIC METHODS

values given in Millions of UollaJ^

•PRODUCTION

a uniform rate of change on a percentage basis. A plotted line representing a uniform rate of increase from year to year on a percentage basis may be seen in the curve given in Fig. 121.

The untrained reader of curves will probably not be able to tell instantly what made the fiat portion of the curve in Fig. 69 during the year 1907 to 1908. One of the chief advantages of the curve method of presenting information is that a curve forces one to think. A little thought here will at once bring out the fact that the flattening of the curve was caused by the 1907 panic. Though the panic started in October of the year 1907, the year 1907 was really one of the most prosperous years the country has ever known. It would be more fitting if the panic were called the 1908 panic, since the main effect of the panic came in 1908 rather than in the year 1907. It can be seen that the Princeton men had their incomes reduced during the year 1908 so that the average fell below that of 1907. By looking along