title: D3 filter() after a selectAll()
body: <p>I'm trying to understand the beatiful library D3. 
  Really – I don't understand why the following lines do not work. 
  <strong>The code should fill only a few of the cirlces in a special color.</strong> </p>
  
  <pre><code>&lt;svg&gt;
  &lt;g id="row_1"&gt; ... &lt;/g&gt;
  &lt;g id="row_1"&gt; ... &lt;/g&gt;
  &lt;g id="row_3"&gt; ... &lt;/g&gt;
      &lt;circle cx="16.887" cy="333.923" r="6.268"/&gt;
      &lt;circle cx="33.574" cy="333.923" r="6.268"/&gt;
      &lt;circle cx="50.262" cy="333.923" r="6.268"/&gt;
      &lt;circle cx="66.949" cy="333.923" r="6.268"/&gt;
      &lt;circle cx="167.074" cy="333.923" r="6.268"/&gt;
      &lt;circle cx="183.762" cy="333.923" r="6.268"/&gt;
      &lt;circle cx="333.387" cy="333.923" r="6.268"/&gt;
      &lt;circle cx="316.699" cy="333.923" r="6.268"/&gt;
      &lt;circle cx="300.199" cy="334.101" r="6.268"/&gt;
      &lt;circle cx="266.637" cy="333.923" r="6.268"/&gt;
      &lt;circle cx="250.137" cy="333.923" r="6.268"/&gt;
      &lt;circle cx="216.762" cy="333.923" r="6.268"/&gt;
  &lt;/g&gt;
  &lt;/svg&gt;
  
  
  &lt;script&gt;
  var i;
  var identifierID;
  var svg = d3.select("svg");  
  
  for (i=0; i&lt;=20; i++){
     identifierID = "#row_"+i;
     svg.select(identifierID).selectAll("circle")
     .filter(function(d){ return d == 12; }).style("fill","blue") *//the value 12 is a example*
  }
  &lt;/script&gt;
  </code></pre>
  
  <p>The code looks like (circle = ° )</p>
  
  <p>° ° ° ° ° ° ° ° ° ° ° (°) &lt;-special color</p>
  
  <p>° ° ° ° ° ° ° ° ° ° ° (°) &lt;-special color</p>
  
  <p>° ° ° ° ° ° ° ° ° ° ° (°) &lt;-special color</p>
  
  <p>But it does not work with the function filter (after all without it does^^).
  It would be great, if someone could help me!
  Regards</p>