title: .attr vs .classed in D3.js
body: <p>In d3, when is it appropriate to use</p>
  
  <pre><code>d3.select("foo").attr('class', 'bar');
  </code></pre>
  
  <p>as opposed to</p>
  
  <pre><code>d3.select("foo").classed('bar', true);
  </code></pre>
  
  <p>?</p>
  
  <p>Is one recommended or expected to be deprecated? What is industry standard?</p>