Groupwise boxplot

Groupwise boxplots can easily be created by means of the formula interface.

boxplot(len ~ supp*dose, data = ToothGrowth,
        main = "Guinea Pigs' Tooth Growth",
        xlab = "Vitamin C dose mg", ylab = "tooth length",
        col=c("yellow", "orange") 
        )

Why an outdated method is described in the boxplot help is however not directly clear. Maybe we are glad to know about the technique anyway someday...

boxplot(len ~ dose, data = ToothGrowth,
       boxwex = 0.25, at = 1:3 - 0.15,
       subset = supp == "VC", col = "yellow",
       main = "Guinea Pigs' Tooth Growth",
       xlab = "Vitamin C dose mg",
       ylab = "tooth length",
       xlim = c(0.5, 3.5), ylim = c(0, 35), yaxs = "i")

boxplot(len ~ dose, data = ToothGrowth, add = TRUE,
       boxwex = 0.25, at = 1:3 + 0.15,
       subset = supp == "OJ", col = "orange")

legend(2, 9, c("Ascorbic acid", "Orange juice"),
      fill = c("yellow", "orange"))