[R] adding mean to boxplot

[Marc Schwartz]

On Thu, 2004-01-22 at 12:11, Johannes Ullrich wrote:
> I am a new and unexperienced user of R and got so far as to know how to produce 
> boxplots. I have no experience of messing with function code, so presently I do 
> not know how to create a boxplot with group means instead of group medians. If 
> somebody could help me either replace the median with the mean or superimpose 
> the mean onto the existing boxplot, it would be appreciated.


You are probably better doing the second option, since boxplots are
premised on the visual display of quantiles.

Here is a quick example of adding means:

# Create two groups of continuous data
# and bind them together
A <- data.frame(Group = "A", 
                Measure = rnorm(50, 5))

B <- data.frame(Group = "B", 
                Measure = rnorm(50, 7.5))
Data <- rbind(A, B)
attach(Data)

# Now create a boxplot with two groups using
# the formula method
boxplot(Measure ~ Group)

# Get the group means
means <- by(Measure, Group, mean)                        

# Plot symbols for each mean, centered on
# x = 1 and x = 2, which are the default center
# values.
points(1:2, means, pch = 23, cex = 0.75,
       bg = "red")

# Now label the means, formatting the values
# to one decimal place. Place the values to the
# left of each group plot.
text(1:2 - 0.4, means, 
     labels = formatC(means, format = "f", 
                      digits = 1),
     pos = 2, cex = 0.9, col = "red")

# Clean up
detach(Data)

See ?boxplot, ?par, ?points, ?text and ?formatC for more information.

You might want to review R News (http://cran.r-project.org/doc/Rnews/)
Vol 3  Number 2 (October 2003), where there is an article providing an
introduction to R's base graphics.

HTH,

Marc Schwartz