The scale for the slanting co-ordinate lines could then be placed at the end of each slanting line at the right-hand side of the chart. The scale for the slanting co-ordinate lines is too difficult to find in Fig. 90.

An interesting study could be made from Fig. 90 by plotting a curve which would show each year the percentage of services which were not metered, instead of using the figures at the top of the chart which show the percentage of services which are metered. A curve

CURVE PLOTTING 97

for the percentages of services not metered should show some similarity in shape to the dotted line curve in Fig. 90, giving the yearly average of daily consumption.

When any curve fluctuates greatly, the general trend of the curve can be most easily determined if the method of moving averages is used. If -data are plotted by months, a moving average is frequently made to include twelve months. As a succeeding month is included in the moving average, that calendar month of the preceding year is dropped out of the average so that the average always includes twelve months. The moving-average curve is a much smoother curve than a curve made from the monthly figures, and is accordingly more easily interpreted. The degree of smoothness of any moving-average curve depends chiefly on the number of points included in the moving average as compared with the number of points in one complete wave or cycle in the data curve, and the moving-average curve is most smooth if the moving average includes the same number of points as are usually found in one complete wave or cycle of the fluctuating curve.