[R] bug with subset and plot?

[Knut Helge Jensen]

Hi!

I have the following problem that I beleive is a bug:

I have a dataframe with one categorical and one numerical vector. The 
categorical vector has three levels (uc, up and vc). A plot of the 
vectors with the categorical vector on the x-axis gives a boxplot with 
three boxes - exactly as expected.

If I then use the subset function to make a dataset that only includes 
two of the levels of the categorical vector and do the plot again, all 
three levels are still shown on the x-axis even though one of them 
doesn't exist in the dataset. The plot shows correct number of boxes (2).

The whole syntax for what I describe is as follows:
exploration.df <- read.table('clipboard', header=T)
attach(exploration.df)
plot(treatment,total.escapes)

exp.df <- subset(exploration.df, treatment!='up')
attach(exp.df)
plot(treatment, total.escapes)

I use R version 2.9.1 (2009-06-26) for Debian.

Knut Helge Jensen