title: Getting attribute values of multiple elements
body: <p>I want to get attribute values of the multiple elements in a selection as an array, but I cannot find a concise way.</p>
  
  <p>For example, in a <code>svg</code> element, there are some <code>circle</code>s and you need to get <code>cx</code> attribute of the all of them.</p>
  
  <p>I tried:</p>
  
  <pre><code>var cxs = d3.select("svg")
      .selectAll("circle")
      .attr("cx");
  </code></pre>
  
  <p>But the result is only a value (say "523.4777243042896") of one circle, and I need all <code>cx</code>s.
  The coordinates of the circles are calculated with a complicated simulation algorithm, so it is impossible to get them using the original data.
  Is there any good way to get all values?</p>
  
  <p>Thanks.</p>