2.22 Add the type of fuel to the plot

Refer to the previous exercise. As we did in Figure 2.6 (page 71), add the categorical variable, type of fuel, to your plot. (If your software does not have this capability, make separate plots for each fuel type. Use the same range of values for the y axis and for the x axis to make the plots easier to compare.) Summarize what you have found in this exercise, and compare your results with what we found in Example 2.7 (pages 70–71).