title: How to make a single plot from two dataframes with ggplot2
body: <p>I have 2 datasets, called A and B.
  I want to compare the distribution of one common variable, called k, showing up in both dataset, but of different lengths (A contains 2000 values of k, while B has 1000, both have some N/A). So I would like to plot the distribution of A$k anf B$k in the same plot.</p>
  
  <p>I have tried:</p>
  
  <pre><code>g1 &lt;- ggplot(A, aes(x=A$k)) + geom_density()
  g2 &lt;- ggplot(B, aes(x=B$k)) + geom_density()
  g &lt;- g1 + g2
  </code></pre>
  
  <p>But then comes the error: </p>
  
  <pre><code>Don't know how to add o to a plot.
  </code></pre>
  
  <p>How can I overcome this problem?</p>