title: geom_point control radius exactly rather than scaling it
body: <p>I have data consisting of a set of circles with radii. The scale of the x, y and radius is the same.</p>
  
  <pre><code>x    y    radius
  0.1  0.8  0.1
  0.4  0.4  0.2
  0.6  0.2  0.9
  0.3  0.6  0.5
  0.5  0.5  0.2
  ...
  0.9  0.1  0.1
  </code></pre>
  
  <p>When I use:</p>
  
  <pre><code>myplot &lt;- ggplot() + geom_point(data=df, aes(x=x, y=y, size=(2*radius)))
  </code></pre>
  
  <p>the resulting plot is a bubble plot with the size scaled to the radius. I want a bubble plot where <code>radius of bubble = radius</code> (i.e. the radius of the bubble is in the native units).</p>
  
  <p>How can I achieve this (in ggplot2)?</p>