title: ggplot combining two plots from different data.frames
body: <p>I want to combine two ggplots, from two different data.frames, into one plot. Below you will find the code. I want to combine plot 1&amp;2 or plots 3&amp;4.</p>
  
  <pre><code>df1 &lt;- data.frame(p=c(10,8,7,3,2,6,7,8),
               v=c(100,300,150,400,450,250,150,400))
  df2 &lt;- data.frame(p=c(10,8,6,4), v=c(150,250,350,400))
  
  plot1 &lt;- qplot(df1$v, df1$p)
  plot2 &lt;- qplot(df2$v, df2$p, geom="step")
  
  plot3 &lt;- ggplot(df1, aes(v, p)) + geom_point()
  plot4 &lt;- ggplot(df2, aes(v, p)) + geom_step()
  </code></pre>
  
  <p>This must be very easy to do, but somehow I can't get it to work. Thanks for your time.</p>