Page:Graphic methods for presenting facts (1914).djvu/109

 Fig. 83 is intended as a comparison between different kinds of coal from the standpoint of actual heating value. At the first glance at this chart the reader sees that the line for Illinois coal is above the other lines, and he is apt to draw the conclusion that Illinois coal is better than anthracite, coke, or Pocahontas coal. It is only after some puzzling over the chart that one notices that the whole chart has been drawn in reversed order. We are considering what the relative values of other coals may be if we know the value of anthracite coal. The whole scheme of reasoning begins with the "price of anthracite coal." The "price of anthracite coal" is the independent variable and should be plotted horizontally, with the "relative value in dollars" plotted as the vertical scale. The reader may get a correct impression of the chart in Fig. 83 if he will turn over the page and read the chart by holding the page up to the light in such manner that the zeros of the two scales appear at the lower left-hand corner. When this is done, it will be seen that the relative value of Pocahontas coal exceeds each of the other fuels mentioned and that Illinois coal comes not at the top of the list, but at the bottom of the list in so far as fuel value is concerned.

Fig. 84 has been drawn with the "price of anthracite coal" as the horizontal scale, where it belongs because it is the independent variable. The lines for different fuels now appear in their correct order, and the reader sees at a glance that Pocahontas coal has more fuel value than anthracite coal. Notice that a heavy line has been used for the curve line drawn for anthracite coal. As this line is the standard by which the values are compared, it seems best to give it prominence on the chart. The position of other curve lines above or below this line shows instantly whether the fuels are better or worse than anthracite in relative value.

In Fig. 85 an effort has been made to show detail time-studies by the use of curves. There is an error here, however, in that the curve has been arranged in such manner that the independent variable is drawn vertically and the information desired as "time in seconds" must be read off from the horizontal scale. The reader wishes to know how many seconds are required for any one step or any series of steps in the whole work. The chart can be interpreted in the accustomed way if the page is turned over, and the diagram read by holding the page in such manner that the zero on the scale of "time in seconds" appears at the lower left-hand corner of the chart when the