Page:Graphic methods for presenting facts (1914).djvu/295

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

The man who plots curves on the cards described here keeps a supply of the printed cards in each of the two rulings of seven spaces, high and ten spaces high. When starting any new curves he uses whichever of the two cards gives the scale best suited to his purpose. The cost of carrying two different kinds of ruled cards on hand is negligible compared with the great convenience resulting.

The ruling of the cards in which the vertical spaces are either 1/4-inch or 1/6-inch high permits the use of an engineer's scale in fortieths or sixtieths of an inch, if it should ever be desired to locate plotted points on the cards with very great accuracy. The engineers' scale in fortieths or sixtieths of an inch gives ten divisions to each space between the horizontal lines on the card and makes it possible to locate each plotted point with a very finely sharpened lead pencil or a needle. Practice, however, proves that there is no necessity for using an engineer's scale in plotting curve points on the cards here described. The man doing the plotting learns very quickly to locate the points by using only the eye and a hard lead-pencil, so that the points are practically as accurate as if spaced with an engineer's scale and a needle point. Even if the points on the curve are not located quite so accurately as they may be when a needle point and a scale are used, it makes no difference to the executive by whom the curve is read. The figures denoting the value of each plotted point are given immediately over the point in the upper margin. The executive reading the curve does not have to refer to the vertical scale. He need only glance at the figures above any point to learn the value for that point far more correctly than would ever be possible with even the most accurately plotted curve if the value of the point had to be interpolated from the vertical scale of the chart. The vertical scale of these cards on which the figures are given in the upper margin fulfills almost no purpose except that of giving a record of the scale to the man who must plot points in succeeding months, or that of giving the values of horizontal lines which are convenient in locating high points or low points on any curve.

When another point has to be added on a large number of curves for a succeeding month or week, the most convenient procedure consists in first copying upon all curve cards, immediately above the vertical line for the proper month, the figures from the typewritten reports, or