Issue with ggplot2, geom_bar, and position=“dodge”: stacked has correct y values, dodged does not

Question

I\'m having quite the time understanding geom_bar() and position=\"dodge\". I was trying to make some bar graphs illustrating two groups. Originall

Answer 1

I think the problem is that you want to stack within values of the num group, and dodge between values of num. It might help to look at what happens when you add an outline to the bars.

library(ggplot2)
set.seed(123)
df <- data.frame(
  id     = 1:18,
  names  = rep(LETTERS[1:3], 6),
  num    = c(rep(1, 15), rep(2, 3)),
  values = sample(1:10, 18, replace=TRUE)
)

By default, there are a lot of bars stacked - you just don't see that they're separate unless you have an outline:

# Stacked bars
ggplot(df, aes(x=factor(names), y=values, fill=factor(num))) + 
  geom_bar(stat="identity", colour="black")

If you dodge, you get bars that are dodged between values of num, but there may be multiple bars within each value of num:

# Dodged on 'num', but some overplotted bars
ggplot(df, aes(x=factor(names), y=values, fill=factor(num))) + 
  geom_bar(stat="identity", colour="black", position="dodge", alpha=0.1)

If you also add id as a grouping var, it'll dodge all of them:

# Dodging with unique 'id' as the grouping var
ggplot(df, aes(x=factor(names), y=values, fill=factor(num), group=factor(id))) + 
  geom_bar(stat="identity", colour="black", position="dodge", alpha=0.1)

I think what you want is to both dodge and stack, but you can't do both. So the best thing is to summarize the data yourself.

library(plyr)
df2 <- ddply(df, c("names", "num"), summarise, values = sum(values))

ggplot(df2, aes(x=factor(names), y=values, fill=factor(num))) + 
  geom_bar(stat="identity", colour="black", position="dodge")