title: How to update multiple charts in d3.js?
body: <p>im researching d3.js over a week already, and I got stuck on one thing.
  I created a donut chart, accroding to this:</p>
  
  <pre><code>http://jsfiddle.net/MX7JC/9/
  </code></pre>
  
  <p>this donut chart has an updae function that uses "arcTween" to update the chart:</p>
  
  <pre><code>function arcTween(a) {
  var i = d3.interpolate(this._current, a);
  this._current = i(0);
  return function(t) {
      return arc(i(t));
  };
  </code></pre>
  
  <p>}</p>
  
  <p>in this "arcTween" function, it uses arc function.
  as it was witten for 1 donut chart, it was globaly declared.
  since im using a multiple charts on page.
  i wonder what is the way to do it.
  i already tried to create this "arc" in an array, but it somehow doesnt work.</p>
  
  <p>if there other way to achieve this, i would be happy to know.</p>