R graphics: add labels to clustered/dodged bar chart

Question

This is a basic plotting question:

I need to add labels to a clustered/dodged bar chart. I have looked at several examples using text(), but cannot seem to position the labels correctly.

teachers <- c("A", "B","C", "D", "E")
mean_pre_scores <- c(10, 11, 12, 10,9)
mean_post_scores <- c(12,15,17,13,12)
pre_post <- data.frame(mean_pre_scores, mean_post_scores)

pre_post <- as.matrix(pre_post)
barplot((t(pre_post)), beside = T, names = teachers, legend = c("pre", "post"), 
        ylim = c(0,20), args.legend = list(x="bottomright"),  axes = T, main = "Unit 1 Test",
        col=c(26,51))

I want to modify this plot so that the values will be displayed above the bars. It would also be helpful to know how to show the values inside the bars.

Answer 1

I think this is what you're after:

z <- barplot((t(pre_post)), beside = T, names = teachers, 
    legend = c("pre", "post"), ylim = c(0,20), 
    args.legend = list(x="topright"),  axes = T, 
    main = "Unit 1 Content Pre Test", col=c(26,51))

text(cex=1, x=c(z[1, ], z[2, ]), y=c(pre_post) + par("cxy")[2]/2, c(pre_post), xpd=TRUE) 

To move the text inside the bars simply use subtractions as in:

text(cex=1, x=c(z[1, ], z[2, ]), y=c(pre_post) - par("cxy")[2]/2, c(pre_post), xpd=TRUE) 

Answer 2

When you say "above the bars", do you mean directly above them? Or on the top axis?

Here's an example with them on top.

And now it's clear you wanted the numeric values on the bars instead of the letters on the top axis. I'll leave it here anyway, and post a second result below.

> barplot((t(pre_post)), beside = T, legend = c("pre", "post"), ylim = c(0,20),
          args.legend = list(x="bottomright"),  axes = T, col=c(26,51))
> axis(3, at = c(2, 5, 8, 11, 14), labels = teachers)
> title(main = "Unit 1 Content Pre Test", line = 3)
> box()

Here's the correct result. I moved the legend to the top right corner where it won't be covering any bars. A bit easier to read that way.

> barplot((t(pre_post)), beside = TRUE, legend = c("pre", "post"), 
          main = "Unit 1 Content Pre Test", ylim = c(0,20), 
          args.legend = list(x="topright"), col=c(26,51))
> text(x = c(1.5,2.5,4.5,5.5,7.5,8.5,10.5,11.5,13.5,14.5), 
       y = txt.height+1, labels = as.character(txt.height))
> box()

Answer 3

A ggplot2 solution:

# get the data into the right format
pre_post <- data.frame(teachers,mean_pre_scores, mean_post_scores)
library(reshape2)
prepost <- melt(pre_post, id = "teachers")

# create the plot
ggplot(prepost, aes(x = teachers, y = value, fill = variable)) +
  geom_bar(stat = "identity", position = "dodge") +
  geom_text(aes(label = value), vjust = -0.5, position = position_dodge(0.9)) +
  theme_bw()

the result:

Answer 4

Adapted from this post (R graphics: Add labels to stacked bar chart)

For inside labels:

teachers <- c("A", "B","C", "D", "E")
mean_pre_scores <- c(10, 11, 12, 10,9)
mean_post_scores <- c(12,15,17,13,12)
pre_post.df <- data.frame(mean_pre_scores, mean_post_scores)

pre_post <- as.matrix(pre_post.df)
b<-barplot((t(pre_post)), beside = T, names = teachers, legend = c("pre", "post"), 
     ylim = c(0,20), args.legend = list(x="bottomright"),  axes = T, main = "Unit 1 Content Pre Test",
     col=c(26,51))


ypos.inside<-apply(pre_post, 2, function(x) x -1 )
ypos.inside <- t(ypos.inside)

text(b, ypos.inside, pre_post)

For outside labels:

teachers <- c("A", "B","C", "D", "E")
mean_pre_scores <- c(10, 11, 12, 10,9)
mean_post_scores <- c(12,15,17,13,12)
pre_post.df <- data.frame(mean_pre_scores, mean_post_scores)

pre_post <- as.matrix(pre_post.df)
b<-barplot((t(pre_post)), beside = T, names = teachers, legend = c("pre", "post"), 
     ylim = c(0,20), args.legend = list(x="bottomright"),  axes = T, main = "Unit 1 Content Pre Test",
     col=c(26,51))

ypos.outside<-apply(pre_post, 2, function(x) x +1 )
ypos.outside <- t(ypos.outside)
text(b, ypos.outside, pre_post)