When plotting curves on large sheets of co-ordinate paper it frequently happens that the scale desired cannot be placed upon the kind of ruled paper available. Scales must usually be made in full size, one-half size, one-quarter size, etc., and it is usually too big a jump to change from one of these sizes to another. The ideal arrangement is to have a supply of co-ordinate paper with different rulings so that when one ruling does not suit, some other ruling may be used. On the curve cards shown in this chapter the horizontal scale is fixed, as the cards are designed for use with definite units of time, such as one year by months, one year by weeks, five years by months, etc. For the vertical scale, however, two different rulings are provided. One of these rulings has seven 1/4-inch spaces as may be seen in Fig. 211. The other ruling has ten vertical spaces of 1/6 inch each as seen in Fig. 209. The ratio of the two scales is seven to ten or, roughly, one scale may be said to be two-thirds of the other. This is the most desirable arrangement that it is possible to get, as these two rulings afford a great range of scales to choose from. If by using one ruling the curve comes too high on the card, simply change over to the other ruling and the peak will come lower down on the card. The two rulings, either in full size or in some fractional size, give every possible combination of scale that need be desired.
The ruling having vertical spaces of one-sixth of an inch is ten spaces high. This ruling is very convenient for curve plotting on a percentage basis when 100 per cent is shown at the top of the chart as in Fig. 126, Fig. 128 and Fig. 129 in Chapter IX. If, on the ten-space card, each space is used to represent 10 per cent with zero for the bottom line and 100 per cent for the top line of the chart, the neatest possible arrangement is secured.
In starting to plot a curve which is to be continued year after year, it is ordinarily best to allow plenty of room for future growth. In the upper curve of Fig. 209 the scale was purposely selected for the first year so that the curve would extend only about one-third of the way to the top of the ruled portion of the card. This would allow the sales to be trebled in volume before the curve would extend over the top of the ruled portion and necessitate a change in scale or a step upward so that the zero line could no longer be shown at the bottom of the card. It is well to start all curves for output or sales at about one-third of the height of the card so as to allow plenty of room for future expansion. Curves for expenses per unit, however,
