title: Fade out part of an HTML 5 canvas after drawing
body: <p>As far as I understand HTML5's canvas doesn't allow access to elements after they are drawn because they immediately become graphic-mode.</p>
  
  <p>I just came across the great demonstration of the HTML5 Audio API at <a href="http://www.storiesinflight.com/jsfft/visualizer/index.html" rel="nofollow">http://www.storiesinflight.com/jsfft/visualizer/index.html</a>, and I'm wondering how the circles shrink and fade out after they are drawn.  (To see, just click pause and watch each arc element fade away into nothing.)  How is this done?  Snippets of the JS would be great.</p>
  
  <p>Many thanks.</p>
  
  <p>Note: I believe Firefox 4 is the only browser this runs on currently.</p>