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.

Joint lines were devised chiefly to permit of showing a large upward or downward progress in the curves for succeeding years without the necessity of laboriously replotting the curves for the earlier years. The desired result has been very satisfactorily attained. Should anyone, however, object to the presenting of a series of cards in steps as in Fig. 211, he need only replot the curve for the earlier years to some smaller scale. In general, however, it will be found that practically no replotting is necessary or desirable. By connecting the cards with joint lines and by using a wavy base line when the scale does not extend to zero, all necessary convenience and accuracy may be secured.

274 GRAPHIC METHODS

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.