If the co-ordinate lines were drawn so as to show the zero line, the base of the chart would be about ^ inch lower than it appears in Fig. 77, and the whole curve would make a different impression. The omission of the zero line in charts of this kind is particerror made by persons

ZDOOO 24 25 26 27 28 29 JO jr I 2

March. 1913 April

Engineering Retard

Fig. 77. Flood in the Hudson River at Mechanicsville, N. Y., March, 1913

Note the different scales at the right and the left by which the curve may be interpreted. This chart is misleading because the scales do not begin at zero

ularly irritating, yet it is a very common drawing charts.

Note that in Fig. 77 the curve can be read from two distinct scales, one scale on the left side of the chart, and a different scale on the

GRAPHIC METHODS

CITIES

right side of the chart. If only one scale is used, it should be placed at the left-hand side of the chart. In very large charts it is sometimes desirable to repeat the scale at the right-hand side as well. Where two different units of measurement are used in the scales, the units should be carefully named so that there will be no danger of the reader's using the right-hand and the left-hand scales interchangeably as though they represented the same unit.

Charts like that shown in Fig. 78 are quite frequently used in public health reports. It is difficult to see how such an unsatisfactory type of chart ever came into general use, unless it was because there are twelve months in a year and twelve hours on the face of a clock. If the death rates for the different months of the year were plotted in a curve, using rectangular coordinates, the data would be just as easy to read and to understand as when shown by the radial scheme (polar co-