title: How to get quantile of object array
body: <p>I have an object array, called <code>dataArray</code>, where one object is like this:</p>
  
  <pre><code>{
     id,
     x,
     y
  }
  </code></pre>
  
  <p>now I want to have the lower quantile of this array by using d3. I thought about something like this:</p>
  
  <pre><code>quartile = d3.quantile(dataArray, 0.25, function(d) {return d.y})
  </code></pre>
  
  <p>but unfortunately there is no accessor option for this method. How can I get the quantile of a property from an object array ? Thanks in advance</p>