title: ggplot's qplot does not execute on sourcing
body: <p>Let's assume I have 2 source files, the first one named <code>example1.r</code> and the second one <code>example2.r</code> (given below).</p>
  
  <p><strong>example1.r</strong></p>
  
  <pre><code>plot(1:10,1:10)
  </code></pre>
  
  <p><strong>example2.r</strong></p>
  
  <pre><code>qplot(1:10,1:10)
  </code></pre>
  
  <p>When I source example1.r, the graph is drawn. It does not, however, when I source example2.r. What is the solution here? </p>
  
  <p>(qplot in example2.r is <a href="http://had.co.nz/ggplot2/" rel="noreferrer">ggplot2</a>'s function)</p>