Whenever zero is not shown at the bottom of the ruled field, this wavy line should be used. Any card can thus be read independently, with safety so far as its interpretation from the zero point is concerned. When several cards are laid out together, and the zero line is shown on the left-hand card, as in Fig. 211, it is a simple matter for the eye to imagine the zero line extended to the right below the other cards, thus permitting easy interpretation of all cards.

It sometimes happens, especially in plotting costs, that the desired direction of the curve will be downward instead of upward. In such a case, and to show small fluctuations from month to month, the scale on the curve cards may be so selected that the zero line does not appear even on the first card plotted. Should the costs be reduced so rapidly that the curve tends to run off the bottom of any card, the card for the succeeding year may have the joint line drawn in such a way as to allow an extension of the scale downward, exactly as Fig. 211 shows an extension of the scale upward. By drawing the left-hand joint line on the later cards above the bottom of the ruled space, and by putting the right-hand joint lines for the first cards at the bottom of the ruled curve field, the series of curves can be made to progress downward to any desired extent in exactly the same manner as the curves in Fig. 211 progress upward year by year. By the use of these joint lines a thoroughly universal arrangement of cards may be secured, allowing extra space for movement either up or down.