R help - Apr 2010 - How to make a boxplot with exclusion of certain groups

This seems like a simple thing, but I have been stuck for some time.  My 
data has 2 columns.  Column 1 is the value, and column 2 is the Site where 
data was collected.  Column 2 contains 7 different Sites (i.e. groups).  I 
am only interested in showing 3 groups on a single boxplot. 

I have tried various methods of subsetting the data, in order to only have 
the 3 groups in my subset.  However even after doing this, all 7 groups 
carry forward, so that when I make a boxplot of my subsetted data, all 7 
groups still appear in the x-axis labels; all 7 groups also appear in the 
boxplot summary (i.e. the values returned with boxplot (…plot=FALSE)  ) . 
Even if I delete the unwanted groups from the ‘levels’ of Column 2, they 
still appear on the plot, and in the boxplot summary statistics.

There are various tricks I can do with the boxplot summary statistics to 
correct for this, but they get complicated when I want to change the 
algorithm for calculating outliers and their corresponding groups. Rather 
than do all these tricks, it seems much simpler to fully exclude the 
unwanted groups from the beginning.  But this doesn’t appear to work

Any ideas?

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On 4/19/2010 12:21 PM, Josef.Kardos at phila.gov wrote:
> This seems like a simple thing, but I have been stuck for some time.  My 
> data has 2 columns.  Column 1 is the value, and column 2 is the Site where 
> data was collected.  Column 2 contains 7 different Sites (i.e. groups).  I 
> am only interested in showing 3 groups on a single boxplot. 
> 
> I have tried various methods of subsetting the data, in order to only have 
> the 3 groups in my subset.  However even after doing this, all 7 groups 
> carry forward, so that when I make a boxplot of my subsetted data, all 7 
> groups still appear in the x-axis labels; all 7 groups also appear in the 
> boxplot summary (i.e. the values returned with boxplot (???plot=FALSE)  ) .
> Even if I delete the unwanted groups from the ???levels??? of Column 2,
they
> still appear on the plot, and in the boxplot summary statistics.
> 
> There are various tricks I can do with the boxplot summary statistics to 
> correct for this, but they get complicated when I want to change the 
> algorithm for calculating outliers and their corresponding groups. Rather 
> than do all these tricks, it seems much simpler to fully exclude the 
> unwanted groups from the beginning.  But this doesn???t appear to work
> 
> Any ideas?
library(gdata) # for drop.levels()

DF <- data.frame(site = rep(LETTERS[1:7], each=20), y = runif(7*20))

boxplot(y ~ drop.levels(site), data=subset(DF, site %in%
c('A','D','F'),
drop=TRUE))
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> 
> 
> 
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