The price of anthracite coal should have been made the horizontal scale of the chart. See Fig. 84

Fig. 82 is an interesting piece of work and the method used in charting is justifiable, even though in this case, as in the preceding, the independent variable is plotted downward and the dependent variable is plotted horizontally.

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Price of Anthracite Cool -- Dollars

Fig. 84. Relative Value of Different Coals as Compared to Anthracite Coal

With the arrangement shown here the curve lines for different coals appear in their correct position. Illinois coal is at the bottom instead of at the top. The heavy line here drawn for anthracite proves at a glance which fuels are better and which poorer than anthracite

CURVE PLOTTING 89

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.