title: How to save a plot by Gadfly.jl without displaying
body: <pre><code>using Gadfly
  myplot = plot(x = 1:10, y = 2:11, Geom.line)
  draw(PNG("myplot.png", 3inch, 3inch), myplot)
  </code></pre>
  
  <p>I just want to make a png file without displaying anything in browsers. (Why? For example, I am working at a remote server.)
  How can I stop the function <code>plot</code> from displaying the figure?</p>