title: Why is my d3.js selectAll + filter not filtering?
body: <p>I'm trying to create a parent <strong>g</strong> tag for each day which will contain a child <strong>g</strong> tag for the label.  The first item section of code produces the parents properly:</p>
  
  <pre><code>var content = d3.select(".content").attr("height", 1000).attr("width", 1000);
  
  //create day groups
  var days = content.selectAll("g").filter(".day")
          .data([0,1,2,3,4,5,6,7,8,9])
          .enter().append("g")
          .attr("class", "day");
  </code></pre>
  
  <p>When I try to add a child <strong>g</strong> tag to each of them, though, it doesn't work:</p>
  
  <pre><code>// create the day headers
  var day_labels = days.selectAll("g").filter(".day_label")    
          .data(function (d) {
              return d;
          })
          .enter().append("g")
          .attr("class", "day_label");
  </code></pre>
  
  <p>Is there something wrong with my syntax?</p>