title: Greek symbols in ggplot2 facet grid labels
body: <p>I want to plot a data frame similar to the following example:</p>
  
  <pre><code>rho_values &lt;- 1:3
  rho0_values &lt;- 4:6
  combinations &lt;- expand.grid(rho_values, rho0_values)
  mydf &lt;- data.frame(rho = rep(combinations[,1], each = 10), rho0 = rep(combinations[,2], each = 10), val = rnorm(90), x = rnorm(90))
  
  p &lt;- ggplot(mydf, aes(x, val)) + geom_point()
  p &lt;- p + facet_grid(rows = vars(rho), cols = vars(rho0), labeller = "label_both")
  p
  </code></pre>
  
  <p>This results in a facet grid plot, where the labels are "rho: 1", "rho: 2", ... in the rows, and "rho0: 4", "rho0: 5", ... in the columns. Instead, I would like to have labels of the form 
  $\rho = 1$, $\rho = 2$, ... in the rows, and $\rho_0 = 4$, $\rho_0 = 5$, ... in the columns (sorry, I am not sure how to use LaTeX here...).</p>
  
  <p>How can I achieve this?</p>