If the zero line of the vertical scale cannot be shown at the bottom of a curve chart, the bottom line should be a slightly wavy line indicating that the field has been broken off and does not reach to zero.

16. When curves are drawn on logarithmically ruled paper, the bottom line and the top line of the chart should each be at some power of ten on the vertical scale.

17. When the scale of a curve chart refers to percentages, the line at 100 per cent should be a broad line of the same width as a zero line.

18. If the horizontal scale for a curve begins at zero, the vertical line at zero (usually the left-hand edge of the field) should be a broad line.

19. When the horizontal scale expresses time, the lines at the leftand right-hand edges of a curve chart should not be made heavy, since a chart cannot be made to include the beginning or the end of time.

20. When curves are to be printed, do not show any more coordinate lines than necessary for the data and to guide the eye. Lines l^:;i^-inch apart are sufficient to guide the eye.

A FEW CAUTIONS 363

21. Make curves with much broader lines than the co-ordinate ruhng so that the curves may be clearly distinguished from the background.

22. Whenever possible have a vertical line of the co-ordinate ruling for each point plotted on a curve so that the vertical lines may show the frequency of the data observations.