title: ggplot ordering a clustered barplot
body: <p>Code to reproduce the issue I have:</p>
  
  <pre><code>library("data.table")
  library("ggplot2")
  DT&lt;-data.table(team=c("Q1","Q2","Q3"), mon=c(3,5,2), tues=c(4,2,1), weds=c(4,2,5))
  DT&lt;-melt(DT,id.vars = "team", measure.name = c("mon","tues","weds"))
  chartdata&lt;-DT[,.(team, day=variable, score=value)]
  ggplot(chartdata, aes(fill=day, y=score, x=team)) +#reorder(data3$Insurer, if(thisdir=="asc") {value} else {-value}))) + 
    geom_bar(position="dodge", stat="identity") 
  </code></pre>
  
  <p>This produces a clustered barplot. I need to set the order by Monday's score (descending), but can't see a way of doing this. I have tried:</p>
  
  <pre><code>ggplot(chartdata, aes(fill=day, y=score, x=reorder(team, {-score}))) + 
    geom_bar(position="dodge", stat="identity") 
  </code></pre>
  
  <p>but this appears to sort the data measured by the totals of Monday - Wedsnesday, not using only Monday as I want.</p>
  
  <p>Is this possible? Many thanks!</p>