title: pass string to facet_grid : ggplot2
body: <p>In ggplot2 you can pass character arguments inside a user defined function using <code>aes_string</code>.  How can you do the same for facet grid which takes a formula, not <code>aes</code>?</p>
  
  <pre><code>FUN &lt;- function(data, x, y, fac1, fac2) {
       ggplot(data = data, aes_string(x=x, y=y)) +
       geom_point() + facet_grid(as.formula(substitute(fac1 ~ fac2)))
  }
  
  
  FUN(mtcars, 'hp', 'mpg', 'cyl', 'am')
  </code></pre>