title: back-to-back barplot with independent axes R
body: <p>I am wanting to plot back-to-back barplot, however each side is on an independent axes. I can plot them back to back by taking the negative of one set, but that leaves them on the same access and because pvalues are smaller their bars are barely represented.     </p>
  
  <pre><code>library(ggplot2)
  df &lt;-structure(list(Description = c("a", "b", "c", "d", "e", "f", 
      "g", "h", "a", "b", "c", "d", "e", "f", "g", "h"), test = c("size", 
      "size", "size", "size", "size", "size", "size", "size", "p", 
      "p", "p", "p", "p", "p", "p", "p"), value = c(0.1, 0.1, 0.125, 
      0.1, 0.075, 0.1, 0.075, 0.125, 0.000230705311441713, 0.000314488619269942, 
      0.00106639822095382, 0.00108290238851994, 0.00114723539549198, 
      0.00160204850890075, 0.0019276388745184, 0.00320371567547557)), .Names = c("Description", 
      "test", "value"), row.names = c(NA, -16L), class = "data.frame")
  
  df$value[df$test == 'p'] &lt;- -(df$value[df$test == 'p'])
  
  ggplot(df, aes(x=Description, y= value, group=test, fill=test)) + geom_col() +coord_flip()
  </code></pre>
  
  <p>Ideally I would like each group on independent axes so that the bars meet at zero (in the middle of the plot region) but be on different scales for this example ylim would be something like ylim(0,0.13) and for pvalue c(0, 0.0035) </p>