[R] Again, about boxplot

[Marc Schwartz]

on 09/12/2008 10:07 AM cathelf wrote:
> Thank you for your guys reply for my previous question. But I got one more
> question about the boxplot. With the code in the R-help:
> 
> boxplot(len ~ dose, data = ToothGrowth,
>         boxwex = 0.25, at = 1:3 - 0.2,
>         subset = supp == "VC", col = "yellow",
>         main = "Guinea Pigs' Tooth Growth",
>         xlab = "Vitamin C dose mg",
>         ylab = "tooth length", ylim = c(0, 35), yaxs = "i")
> boxplot(len ~ dose, data = ToothGrowth, add = TRUE,
>         boxwex = 0.25, at = 1:3 + 0.2,
>         subset = supp == "OJ", col = "orange")
> legend(2, 9, c("Ascorbic acid", "Orange juice"),
>        fill = c("yellow", "orange"))
> 
> I got 6 boxplots, which is ordered as "0.5, 0.5, 1, 1, 2, 2"
> How can I reorder the 6 boxplots as "0.5, 1, 2,  0.5, 1, 2"?
> 
> Thank you very much!

Here is one approach:

# Create a new DF, adding a column with the interaction of supp and
# dose.
DF <- cbind(ToothGrowth,
            SD = interaction(ToothGrowth$supp, ToothGrowth$dose,
                             lex.order = TRUE))

# Note DF$SD and the ordering of the factor levels, which is
# the order of the boxes in boxplot()
> DF$SD
 [1] VC.0.5 VC.0.5 VC.0.5 VC.0.5 VC.0.5 VC.0.5 VC.0.5 VC.0.5 VC.0.5
[10] VC.0.5 VC.1   VC.1   VC.1   VC.1   VC.1   VC.1   VC.1   VC.1
[19] VC.1   VC.1   VC.2   VC.2   VC.2   VC.2   VC.2   VC.2   VC.2
[28] VC.2   VC.2   VC.2   OJ.0.5 OJ.0.5 OJ.0.5 OJ.0.5 OJ.0.5 OJ.0.5
[37] OJ.0.5 OJ.0.5 OJ.0.5 OJ.0.5 OJ.1   OJ.1   OJ.1   OJ.1   OJ.1
[46] OJ.1   OJ.1   OJ.1   OJ.1   OJ.1   OJ.2   OJ.2   OJ.2   OJ.2
[55] OJ.2   OJ.2   OJ.2   OJ.2   OJ.2   OJ.2
Levels: OJ.0.5 OJ.1 OJ.2 VC.0.5 VC.1 VC.2

# Now do the boxplot
boxplot(len ~ SD, data = DF, boxwex = 0.25, at = c(1:3, 5:7),
        xlim = c(1, 7), col = c(rep("orange", 3), rep("yellow", 3)))

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


See ?interaction for more information.

HTH,

Marc Schwartz