title: ggplot ordering facet labels by value
body: <p>With a data frame <code>df</code> like below, I'm plotting bar plots with faceting.</p>
  
  <pre><code>text &lt;- "
  make,var,value
  fiat,mileage,2.1
  astom,mileage,1.8
  fiat,disp,1.4
  astom,disp,1.7
  "
  df &lt;- read.table(textConnection(text), sep = ",", header = TRUE)
  
  ggplot(df, aes(x=make, y=value) ) +
    geom_bar(stat = 'identity') +
    facet_wrap(~ var, scales = "free", ncol=1)
  </code></pre>
  
  <p>This gives a plot like below.</p>
  
  <p><a href="https://i.stack.imgur.com/RJLha.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RJLha.png" alt="enter image description here"></a></p>
  
  <p>However, I want x-axis labels to be ordered in descending order of the value of the <code>var</code>- in the example above for the <code>mileage</code> var, <code>fiat</code> should be shown before <code>astom</code>  - how do I got about it ? </p>