title: adjust text in the center with position dodge
body: <p>Using this code </p>
  
  <pre><code>library(ggplot2)
  ggplot( dat,aes(x=factor(group),y=difference) ) +
    geom_bar( width=.75,aes(fill=as.factor(Mutation)),stat="identity",
              position="dodge") +
    geom_text(aes(label=Mutation),position=position_dodge(height=0.9),angle=90)+
    theme_bw() + 
    theme(legend.position="none")
  </code></pre>
  
  <p>I generate the flowing plot. As you see some text are not in the center of the bars. I tried some options like <code>hjust</code> , <code>vjust</code> or playing with <code>dodge width</code>, without success. Can someone help me get the text in the center of bars?</p>
  
  <p>In the plot below we have problem with group = 1 ,3,8,9,...</p>
  
  <p><img src="https://i.stack.imgur.com/yiL1x.png" alt="enter image description here"></p>
  
  <p>Here my data : </p>
  
  <pre><code>dat &lt;-  structure(list(Mutation = structure(1:21, .Label = c("2rN7y", 
  "2wTys", "8ElEj", "8esgR", "8ppm9", "BkuXt", "bM5sv", "c7KTn", 
  "dnBl6", "F5nhO", "fFpM6", "hMpSW", "HxAh9", "KDI0t", "qZTSa", 
  "rOONr", "Tf2kX", "TGDZE", "tLdje", "XsGz6", "XTGN1"), class = "factor"), 
      difference = c(2.1499193357556, 0.27602347826589, -0.22889581513082, 
      -0.77297822818092, -0.468200478640476, -0.735026155784277, 
      -0.900791750580477, -1.33435362820732, 0.412022274758476, 
      0.495870479997156, 0.793585307678721, 1.22839278213642, -1.0700682293443, 
      0.63436212480624, -0.152410633064764, 1.23397624015726, -0.520869343832941, 
      1.62320252173067, -0.24168976773895, -1.04897550447309, 1.68588724420516
      ), position = c(4, 4, 3, 2, 3, 3, 2, 3, 2, 5, 2, 2, 2, 5, 
      5, 5, 2, 4, 5, 4, 2), group = c(0L, 0L, 1L, 2L, 3L, 3L, 4L, 
      5L, 6L, 7L, 8L, 8L, 8L, 9L, 9L, 9L, 10L, 11L, 12L, 13L, 14L
      )), .Names = c("Mutation", "difference", "position", "group"
  ), row.names = c(11L, 2L, 19L, 10L, 9L, 21L, 20L, 12L, 16L, 13L, 
  17L, 1L, 6L, 15L, 18L, 14L, 4L, 5L, 8L, 3L, 7L), class = "data.frame")
  </code></pre>