title: I can tween a D3 arc's endAngle, but not it's startAngle. What am I doing wrong?
body: <p>I'm just playing with this demo and recreating it myself: <a href="http://bl.ocks.org/mbostock/5100636" rel="nofollow">http://bl.ocks.org/mbostock/5100636</a></p>
  
  <p>I can define a new endAngle and it'll animate just fine, but now I'm trying to animate the startAngle too. I just can't get it to work.</p>
  
  <pre><code>function animateArc(newStart, newEnd) {
  
      myArc.transition().duration(750).call(arcTween, newStart, newEnd);
  
      function arcTween(transition, newStart, newEnd) {
          transition.attrTween("d", function(d) {
              var interpolateStart = d3.interpolate(d.startAngle, startAngle);
              var interpolateEnd = d3.interpolate(d.endAngle, endAngle);
              return function(t) {
                  d.startAngle = interpolateStart(t);
                  d.endAngle = interpolateEnd(t);
                  return arc(d);
              };
          });
      }
  }
  </code></pre>
  
  <p>I get no errors of any kind, and the endAngle still animates, but the startAngle code I added is literally just.. ignored.</p>
  
  <p>What is the correct way to tween an arc's startAngle?</p>