title: Side-by-side plots with ggplot2
body: <p>I would like to place two plots side by side using the <a href="http://crantastic.org/packages/ggplot2" rel="noreferrer">ggplot2 package</a>, i.e. do the equivalent of <code>par(mfrow=c(1,2))</code>.</p>
  
  <p>For example, I would like to have the following two plots show side-by-side with the same scale.</p>
  
  <pre><code>x &lt;- rnorm(100)
  eps &lt;- rnorm(100,0,.2)
  qplot(x,3*x+eps)
  qplot(x,2*x+eps)
  </code></pre>
  
  <p>Do I need to put them in the same data.frame?</p>
  
  <pre><code>qplot(displ, hwy, data=mpg, facets = . ~ year) + geom_smooth()
  </code></pre>