You can use the following methods to draw a boxplot with a mean value in R:

Method 1: Use Base R

#create boxplots
boxplot(df$values~df$group)

#calculate mean value by group
means <- tapply(df$values, df$group, mean)

#add means as circles to each boxplot
points(means, pch=20) 

Method 2: Use ggplot2

library(ggplot2)

#create boxplots with mean values shown as circles
ggplot(df, aes(x=group, y=values, fill=group)) +
  geom_boxplot() +
  stat_summary(fun=mean, geom='point', shape=20)

The following examples show how to use each method in practice with the following data frame in R:

#create data frame
df <- data.frame(team=rep(c('A', 'B', 'C'), each=5),
                 points=c(4, 4, 5, 6, 8, 7, 6, 8, 9, 12,
                          11, 12, 13, 16, 18))

#view first six rows of data frame
head(df)

  team points
1    A      4
2    A      4
3    A      5
4    A      6
5    A      8
6    B      7

Example 1: Create Boxplots with Mean Values in Base R

The following code shows how to create boxplots with mean values in base R:

#create boxplots
boxplot(df$points~df$team)

#calculate mean value by group
means <- tapply(df$points, df$team, mean)

#add means as circles to each boxplot
points(means, pch=20, cex=1.5)

The black lines inside each boxplot represent the median value and the black circles inside each boxplot represent the mean value.

Note: Change the value for the cex argument to change the circle size.

Example 2: Create Boxplots with Mean Values in ggplot2

The following code shows how to create boxplots with mean values in ggplot2:

library(ggplot2)

#create boxplots with mean values
ggplot(df, aes(x=team, y=points, fill=team)) +
  geom_boxplot() +
  stat_summary(fun=mean, geom='point', shape=20, size=8) +
  theme(legend.position='none')

Note: Change the value for the size argument within the stat_summary() function to change the circle size.

Additional Resources

The following tutorials offer additional information about boxplots: