stat_sum and stat_identity give weird results

Question

I have the following code, including randomly generated demo data:

n <- 10
group <- rep(1:4, n)
mass.means <- c(10, 20, 15, 30)
mass.sigma <- 4
score.means <- c(5, 5, 7, 4)
score.sigma <- 3
mass <- as.vector(model.matrix(~0+factor(group)) %*% mass.means) +
  rnorm(n*4, 0, mass.sigma)
score <- as.vector(model.matrix(~0+factor(group)) %*% score.means) +
  rnorm(n*4, 0, score.sigma)
data <- data.frame(id = 1:(n*4), group, mass, score)
head(data)

Which gives:

  id group      mass    score
1  1     1 12.643603 5.015746
2  2     2 21.458750 5.590619
3  3     3 15.757938 8.777318
4  4     4 32.658551 6.365853
5  5     1  6.636169 5.885747
6  6     2 13.467437 6.390785

And then I want to plot the sum of "score", grouped by "group", in a bar chart:

plot <- ggplot(data = data, aes(x = group, y = score)) + 
  geom_bar(stat="sum") 
plot

This gives me:

Weirdly, using stat_identity seems to give the result I am looking for:

plot <- ggplot(data = data, aes(x = group, y = score)) + 
  geom_bar(stat="identity") 
plot

Is this a bug? Using ggplot2 1.0.0 on R

platform       x86_64-pc-linux-gnu         
arch           x86_64                      
os             linux-gnu                   
system         x86_64, linux-gnu           
status                                     
major          3                           
minor          1.2                         
year           2014                        
month          10                          
day            31                          
svn rev        66913                       
language       R                           
version.string R version 3.1.2 (2014-10-31)
nickname       Pumpkin Helmet    

Or what am I doing wrong?

Answer 1

plot <- ggplot(data = data, aes(x = group, y = score)) + 
  stat_summary(fun.y = "sum", geom = "bar", position = "identity")
plot

aggregate(score ~ group, data=data, FUN=sum)
#  group    score
#1     1 51.71279
#2     2 58.94611
#3     3 67.52100
#4     4 39.24484

Edit:

stat_sum does not work, because it doesn't just return the sum. It returns the "number of observations at position" and "percent of points in that panel at that position". It was designed for a different purpose. The docs say " Useful for overplotting on scatterplots."

stat_identity (kind of) works because geom_bar by default stacks the bars. You have many bars on top of each other in contrast to my solution that gives you just one bar per group. Look at this:

plot <- ggplot(data = data, aes(x = group, y = score)) + 
  geom_bar(stat="identity", color = "red") 
plot

Also consider the warning:

Warning message:
Stacking not well defined when ymin != 0