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 drawn that the line at the right of the 1910 card was above zero, and the joint line at the left of the 1911 card at the bottom of the ruled field. Thus, when the two cards are overlapped so that the joint lines match, it is seen that there is enough space in the ruled field at the right for the curve to rise to the scale line for 1,600. Two more spaces were again allowed in 1912 in anticipation of even further upward progress in the curve. As joint lines may be drawn in continuation of any horizontal line on either the upper or the lower card, any amount of expansion in future years may be provided for. The curve cards are small enough to be very convenient to handle and yet they may be joined together in such a way as to provide for unlimited future growth.

Fig. 211. Total Output in Tons of a Factory for Three Consecutive Years

When a curve shows a tendency to leave the ruled field of the card, more space can be allowed as a result of the way in which cards for succeeding years are joined