title: Overlaying two plots using ggplot2 in R
body: <p>There are two data frames - <code>df1</code> &amp; <code>df2</code> from which I need to create line plot using ggplot2 and display on the same graph.</p>
  <h1><code>df1</code></h1>
  <pre><code> x   y 
  2   30
  4   25
  6   20
  8   15
  </code></pre>
  <h1><code>df2</code></h1>
  <pre><code> x   y 
  2   12
  4   16
  6   20
  8   24
  </code></pre>
  <p>for plotting the first df,</p>
  <pre><code>p1 &lt;- ggplot( df1, aes(x = df1[,1] , y = log(df1[,2]) ) )
  p1 &lt;- p1 + geom_line(aes(y = log(df1[,2])))
  p1
  </code></pre>
  <p>how do we get a same plot overlay on same graph for data frame <code>df2</code></p>