title: d3.scale.category20b always returning first color
body: <p>I try to use d3.scale.category.20b() to generate a color scale, problem is whatever number of the list I ask for, it always returns first element of the list.</p>
  
  <pre><code>var color = d3.scale.category20b();
  console.log(color(X));
  </code></pre>
  
  <p>OR</p>
  
  <pre><code>console.log(d3.scale.category20b()(X);
  </code></pre>
  
  <p>No matter what X is, it always logs #393b79 which is the first elements, according to the <a href="https://github.com/mbostock/d3/wiki/Ordinal-Scales#wiki-category20c" rel="noreferrer">d3 API</a></p>