Residual plots for a output model of class gafem. Seven types
of plots are produced: (1) Residuals vs fitted, (2) normal Q-Q plot for the
residuals, (3) scale-location plot (standardized residuals vs Fitted Values),
(4) standardized residuals vs Factor-levels, (5) Histogram of raw residuals
and (6) standardized residuals vs observation order, and (7) 1:1 line plot.