title: stat_sum and stat_identity give weird results
body: <p>I have the following code, including randomly generated demo data:</p>
  
  <pre><code>n &lt;- 10
  group &lt;- rep(1:4, n)
  mass.means &lt;- c(10, 20, 15, 30)
  mass.sigma &lt;- 4
  score.means &lt;- c(5, 5, 7, 4)
  score.sigma &lt;- 3
  mass &lt;- as.vector(model.matrix(~0+factor(group)) %*% mass.means) +
    rnorm(n*4, 0, mass.sigma)
  score &lt;- as.vector(model.matrix(~0+factor(group)) %*% score.means) +
    rnorm(n*4, 0, score.sigma)
  data &lt;- data.frame(id = 1:(n*4), group, mass, score)
  head(data)
  </code></pre>
  
  <p>Which gives:</p>
  
  <pre><code>  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
  </code></pre>
  
  <p>And then I want to plot the sum of "score", grouped by "group", in a bar chart:</p>
  
  <pre><code>plot &lt;- ggplot(data = data, aes(x = group, y = score)) + 
    geom_bar(stat="sum") 
  plot
  </code></pre>
  
  <p>This gives me: 
  <img src="https://i.stack.imgur.com/OMvZo.png" alt="enter image description here"></p>
  
  <p>Weirdly, using <code>stat_identity</code> seems to give the result I am looking for:</p>
  
  <pre><code>plot &lt;- ggplot(data = data, aes(x = group, y = score)) + 
    geom_bar(stat="identity") 
  plot
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/HRw3A.png" alt="enter image description here"></p>
  
  <p>Is this a bug? Using ggplot2 1.0.0 on R </p>
  
  <pre><code>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    
  </code></pre>
  
  <p>Or what am I doing wrong? </p>