Usually, of course, the zero line is at the bottom of the chart unless there are negative quantities so that the curve crosses over the zero line and extends below it. In all cases the zero line can be made a heavy line. If the curve should extend below the zero line the width of the zero line should be ' so great that the reader will be certain to interpret the chart from the zero line rather than from the bottom line of the chart itself.

It sometimes happens that the data for a chart involve high numerical figures so that a large amount of space must be used if the zero line of the vertical scale is to be shown in the final illustration. In such a case, the bottom of the chart may have a wavy line as seen in Fig. 245 which portrays the same data as Fig. 244. Fig. 244 could have been extended so that the bottom line would be the zero line if a

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Fig. 245. Average per Capita of Total Savings-bank Deposits in the United States

Whenever possible a chart containing curves should be so drawn that the zero of the vertical scale appears in the chart. If the zero line is not shown on the chart, that fact should be indicated by a wavy line at the bottom warning the reader that interpretation must be made from the vertical scale and not by visual measurement from the bottom line of the chart