title: D3 Area chart that tends to zero where values are missing
body: <p>I have data like this: </p>
  
  <pre><code>document: [{"key":"01/01/2001","values":2},
   {"key":"02/01/2001","values":1},
   {"key":"31/01/2001","values":2}]
  </code></pre>
  
  <p>I am creating an area chart with .interpolate("linear") to create the following:
  The idea being to represent number of documents "created" throughout January.</p>
  
  <p><img src="https://i.stack.imgur.com/noULh.jpg" alt="enter image description here"></p>
  
  <p>However, this is kind of a misleading output as it would imply there are values throughout January, when there aren't just 2 at the start and one at the end.</p>
  
  <p>My questions are: </p>
  
  <ol>
  <li>Fundamentally is this the wrong graph to represent this data, and should a bar chart be used instead?</li>
  <li>Can D3 add evenly spaced "zero" values for each day in January?</li>
  <li>Is the best we can do is use .interpolate("cardinal") to produce something like:</li>
  </ol>
  
  <p><img src="https://i.stack.imgur.com/wFcdZ.jpg" alt="enter image description here"></p>
  
  <p>Thanks in advance!</p>