title: x and y variables are from different data frame (ggplot2)
body: <p>I'm learning ggplot2 so please excuse my novice understanding.</p>
  
  <p>Is there a way to do a simple scatter plot when x and y are from two different data frames? For example:</p>
  
  <pre><code>p &lt;- ggplot(data=df1, aes(x=x.in.df1))
  p + geom_point(data=df2, aes(y=y.in.df2))
  </code></pre>
  
  <p>Since <code>x.in.df1</code> does not exist in df2, when geom_point tries to look for <code>x</code> in <code>df2</code>, it fails.</p>
  
  <p>EDIT: The reason why I hesitate to <code>cbind</code> is because I have many data frames with the same variables. For example, df2000 contains variables GDP, GDP growth for all countries in year 2000. df2001 is the same for year 2001.</p>