R help - Apr 2006 - Boxplot using Formula

I am trying to use the formula interface for the boxplot.
Currently running R 2.2.1 on Windows XP.

The problem is that boxplot is displaying groups that are empty in the  
plot.

The following example demonstrates what it is happening (though my actual  
situation is a little more complicated):
> data<-data.frame(values=c(1:25),
groups=rep(c("A","B","C","D","E"),
> each=5))
> boxplot(data$values~data$groups, subset=data$groups!="C")
The plot includes a space for level "C" with no boxplot.  I would like
to
have only those levels with associated boxplots be shown in the figure.   
So in this case, there should only be values "A", "B",
"D", and "E" evenly
spaced along the horizontal axis.

I am a little confused because I am reusing old code and I don't remember  
having this issue before, so perhaps there have been some changes in the  
behavior of the boxplot function since the last time I use the code.

It seems like there's probably a simple solution, but I'm not seeing it.

Thanks,

Matt Goff

Matt Goff <goff <at> nawwal.org> writes:
> 
> 
> The problem is that boxplot is displaying groups that are empty in the  
> plot.
> 
Call factor() again on the groups, which will drop levels. You can do that in an
extra line, or on-the-fly:

data<-data.frame(values=c(1:25),
groups=rep(c("A","B","C","D","E"),
 each=5))
data = subset(data,groups!="C")

boxplot(values~factor(groups), data=data)

Matt Goff wrote:
> I am trying to use the formula interface for the boxplot.
> Currently running R 2.2.1 on Windows XP.
> 
> The problem is that boxplot is displaying groups that are empty in the  
> plot.
> 
> The following example demonstrates what it is happening (though my actual  
> situation is a little more complicated):
> 
> 
>>data<-data.frame(values=c(1:25),
groups=rep(c("A","B","C","D","E"),
>>each=5))
>>boxplot(data$values~data$groups, subset=data$groups!="C")
> 
> 
> The plot includes a space for level "C" with no boxplot.  I would
like to
> have only those levels with associated boxplots be shown in the figure.   
> So in this case, there should only be values "A", "B",
"D", and "E" evenly
> spaced along the horizontal axis.
> 
> I am a little confused because I am reusing old code and I don't
remember
> having this issue before, so perhaps there have been some changes in the  
> behavior of the boxplot function since the last time I use the code.
> 
> It seems like there's probably a simple solution, but I'm not
seeing it.
> 
> Thanks,
> 
> Matt Goff
> 
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Hi, Matt,

I'm afraid you cannot do what you want with boxplot.formula. You have at 
least two options:

1) Supply only data that has no empty levels.
2) Create a new function that does what you need:

## add `drop.unused.levels' argument to boxplot.formula
boxplot2 <-
function (formula, data = NULL, ...,
           subset, na.action = NULL,
           drop.unused.levels = TRUE)
{
     if (missing(formula) || (length(formula) != 3))
         stop("'formula' missing or incorrect")
     m <- match.call(expand.dots = FALSE)
     if (is.matrix(eval(m$data, parent.frame())))
         m$data <- as.data.frame(data)
     m$... <- NULL
     m$na.action <- na.action
     m$drop.unused.levels <- drop.unused.levels
     m[[1]] <- as.name("model.frame")
     mf <- eval(m, parent.frame())
     response <- attr(attr(mf, "terms"), "response")
     boxplot(split(mf[[response]], mf[-response]), ...)
}

values <- 1:25
groups <-
factor(rep(c("A","B","C","D","E"),
each=5))
boxplot2(values ~ groups, subset = groups != "C")

HTH,

--sundar