Figure 8.5f is different from the others because it is not a plot of residuals but rather a plot of the response versus the fitted values. This is really just a rescaling of Figure 8.5e. If the mean function and the assumptions appear to be sensible, then this figure is a summary graph for the regression problem. The mean function for the graph should be the straight line shown in the figure. which is just the fitted OLs simple regression fit of the response on the filled values