Plots a scatter plot for the variables of the residuals and fitted values from the linear model, lmfit. A lowess smooth line for the underlying trend, as well as one standard deviation error bounds for the scatter about this trend, are added to this scatter plot. A test for a quadratic relationship between the residuals and the fitted values is also computed.
an lm object, i.e. the output from "lm()".
the smoother span. This gives the proportion of points in the plot which influence the smooth at each value. Larger values give more smoothness.
Returns the plot.
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