title: d3 scale: How to use .copy()?
body: <p>I came across the <a href="https://github.com/mbostock/d3/wiki/Quantitative-Scales#log_copy" rel="noreferrer"><code>log.copy()</code></a> in the <a href="https://github.com/mbostock/d3/wiki/Quantitative-Scales" rel="noreferrer">API reference</a>. What is the purpose of this? Can someone show an example?</p>
  
  <p>If I use a scale in one function, can I save it to a global variable by using <code>.copy()</code> and retrieve it later?</p>