title: Use for loop to plot multiple lines in single plot with ggplot2
body: <p>I try to plot multiple lines in single plot as follow:</p>
  
  <pre><code>y &lt;- matrix(rnorm(100), 10, 10)
  m &lt;- qplot(NULL)
  for(i in 1:10) {
      m &lt;- m + geom_line(aes(x = 1:10, y = y[,i]))
  }
  plot(m)
  </code></pre>
  
  <p>However, it seems that <code>qplot</code> will parse <code>m</code> during <code>plot(m)</code> where <code>i</code> is <code>10</code>, so <code>plot(m)</code> produces single line only.</p>
  
  <p>What I expect to see is similar to:</p>
  
  <pre><code>plot(1,1,type='n', ylim=range(y), xlim=c(1,10))
  for(i in 1:10) {
      lines(1:10, y[,i])
  }
  </code></pre>
  
  <p>which should contain 10 different lines.</p>
  
  <p>Is there <code>ggplot2</code> way to do this?</p>