title: Issue with geom_text when using position_dodge
body: <p>I saw <a href="https://stackoverflow.com/questions/6017460/ggplot-geom-text-with-dodged-barplot">this</a> answer but couldn't replicate it.</p>
  
  <p>I get my data like this:</p>
  
  <pre><code>df = data.frame(x = rep(sample(letters, 4), 2), 
                  y = round(runif(8,1,100),0), 
                  z = c(rep("group1",4), rep("group2",4)))
  
  # I then add a 'percent' column like so:
  
  df$perc[1:4] = df$y[1:4] / sum(df$y[1:4])
  df$perc[5:8] = df$y[5:8] / sum(df$y[5:8])
  
  # Which I then convert like so:
  df$perc = paste(round(df$perc * 100, 1), "%", sep="")
  
  # The ggplot:
  library(ggplot2)
  
  ggplot(df) + 
  geom_bar(aes(z, y, fill=x), position="dodge", stat="identity") + 
  geom_text(aes(z,y,label=perc), position=position_dodge(width=1), size=4)
  </code></pre>
  
  <p>Result: </p>
  
  <p><img src="https://i.stack.imgur.com/I27Dc.png" alt="enter image description here"></p>
  
  <p>I can't figure out what I did wrong.</p>