title: controlling order of facet_grid/facet_wrap in ggplot2?
body: <p>I am plotting things using <code>facet_wrap</code> and <code>facet_grid</code> in ggplot, like:</p>
  
  <pre><code>ggplot(iris) + geom_histogram(aes(iris$Petal.Width)) + facet_grid(Species ~ .)
  </code></pre>
  
  <p>Is it possible to control the order in which the <code>Species</code> panels are ordered in the plot?  Can this be done without changing the <code>iris</code> dataframe or making a new one?  The default here shows setosa, versicolor, virginica but I'd like a different order. thanks.</p>