title: Multiple variables in geom_bar or geom_col
body: <pre><code>Site TreesSDI InvertebratesSDI PlantsSDI BirdsSDI
  1     1.31      2.48           1.35      1.84
  2     1.46      2.80           1.61      2.05
  3     1.78      2.50           2.29      2.40
  </code></pre>
  
  <p>I have this data recording Shannon's Diversity Index over three sites for trees, inverts, plants and birds. I have tried to get them on the same plot doing this but nothing is working. </p>
  
  <pre><code>ggplot(data) + 
    geom_bar(aes(x= data$Site, y=data$TreesSDI), stat= "identity" , position="dodge") + 
    geom_bar(aes(x= data$Site, y=data$InvertebratesSDI), stat= "identity" , position="dodge") + 
    geom_bar(aes(x= data$Site, y=data$PlantsSDI), stat= "identity" , position="dodge") + 
    geom_bar(aes(x= data$Site, y=data$BirdsSDI), stat= "identity" , position="dodge") +
    ggtitle("Shannon's Diversity of Trees, Invertebrates, Plants and Birds for all Three Sites") + 
    xlab("") + 
    ylab("Shannon's Diversity Index") 
  </code></pre>