title: ggplot scatterplot points with no fill
body: <p>I want to make a scatterplot whose points have no fill (or equivalently, with a transparent fill).</p>
  
  <pre><code># generate some random data for the scatterplot
  n &lt;- 5
  f &lt;- factor(1:n)
  df &lt;- expand.grid(f1 = f, f2 = f)
  df &lt;- transform(df, v1 = round(10 * runif(n ** 2)))
  
  # plot the scatterplot
  ggplot(df) + geom_point(aes(x = f1, y = f2, size = v1, fill = NA))
  </code></pre>
  
  <p>Setting <code>fill</code> to <code>NA</code> seems logical but did not work. I also tried <code>NULL</code> and <code>""</code> to no avail.</p>