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 tenspace 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.

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may be started well up on the card if there is any hope whatever of reducing the unit expenses in future years.