title: Switch placement of facet_grid labels
body: <p>How can I change the placement of the labels of <code>facet_grid()</code> without switching the formula?</p>
  
  <pre><code>library(plyr)
  library(ggplot2)
  
  data = data.frame(id = 1:10, value = rnorm(100*10))
  data = ddply(data, .(id), transform, obs = 1:100)
  
  ggplot(data = data, aes(x = obs, y = value, color = factor(id))) + 
    geom_line() +
    facet_grid(id ~.)
  </code></pre>
  
  <p>In the following picture, I want the panel text to switch from the right to the top of the panel.</p>
  
  <p><img src="https://i.stack.imgur.com/6NcqW.png" alt="enter image description here"></p>