title: What does this.__chart__ refer to in a d3 selection?
body: <p>I've been poking around in <a href="https://github.com/d3/d3-plugins/blob/master/box/box.js" rel="nofollow">d3.box</a> and <a href="https://github.com/d3/d3-plugins/blob/master/bullet/bullet.js" rel="nofollow">d3.bullet</a>. In both cases, the current scale is retrieved using something like the following...</p>
  
  <pre><code>var x0 = this.__chart__ || d3.scale.linear()
    .domain([0, Infinity])
    .range(x1.range());
  </code></pre>
  
  <p>...where <code>x1</code> is the current scale object. I realise that this idiom is used to update the box plot or bullet chart, but could anyone explain how? What does <code>this.__chart__</code> refer to? A scale object? Why the conditional? (<code>||</code>) Why does <code>x0</code>'s domain cover the range 0 to infinity when the current scale is unlikely to have such a large range?</p>
  
  <p>Apologies if my questions are poorly specified. Any help would be greatly appreciated.</p>