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 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.

United States Statistical Atlas of the 1900 Census

Fig. 78. Death Rate from Consumption per 1000 Inhabitants for Each Month of the Year in Cities of the United States

This type of chart should be banished to the scrap heap. Charts on rectangular ruling are easier to draw and easier to understand

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 co-*ordinates, the data would be just as easy to read and to understand as when shown by the radial scheme (polar co-*ordinates) of Fig. 78. There would be the additional advantage that the rectangular method would be more widely understood than the circular method of Fig. 78.

Though a chart in the form of Fig. 79 might be justified in the Sunday supplement of a newspaper where an untrained audience must be reached, it is much better to use a curve in the form of Fig. 80 whenever a trained audience is assured. The most interesting thing about Fig. 79 is the slanting line which gives an unusual optical illusion if observed under artificial light, especially with a bare gas flame. The slanting line then appears blue, although it is printed black like the horizontal bars of the chart.

The dotted line in Fig. 80 corresponds to the slanting line of Fig. 79, and represents a progressive average of all the points on the curve