title: Consistent width for geom_bar in the event of missing data
body: <p>Is there a way to set a constant width for <code>geom_bar()</code> in the event of missing data in the time series example below?  I've tried setting <code>width</code> in <code>aes()</code> with no luck. Compare May '11 to June '11 width of bars in the plot below the code example.</p>
  
  <pre><code>colours &lt;- c("#FF0000", "#33CC33", "#CCCCCC", "#FFA500", "#000000" )
  iris$Month &lt;- rep(seq(from=as.Date("2011-01-01"), to=as.Date("2011-10-01"), by="month"), 15)
  
  colours &lt;- c("#FF0000", "#33CC33", "#CCCCCC", "#FFA500", "#000000" )
  iris$Month &lt;- rep(seq(from=as.Date("2011-01-01"), to=as.Date("2011-10-01"), by="month"), 15)
  d&lt;-aggregate(iris$Sepal.Length, by=list(iris$Month, iris$Species), sum)
  d$quota&lt;-seq(from=2000, to=60000, by=2000)
  colnames(d) &lt;- c("Month", "Species", "Sepal.Width", "Quota")
  d$Sepal.Width&lt;-d$Sepal.Width * 1000
  g1 &lt;- ggplot(data=d, aes(x=Month, y=Quota, color="Quota")) + geom_line(size=1)
  g1 + geom_bar(data=d[c(-1:-5),], aes(x=Month, y=Sepal.Width, width=10, group=Species, fill=Species), stat="identity", position="dodge") + scale_fill_manual(values=colours)
  </code></pre>
  
  <p><img src="https://i.imgur.com/TwPle.png" alt="plot"></p>