adding mean to boxplot

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