title: d3 append and enter issues
body: <p>So I am having a problem when following a very simple d3 tutorial. Essentially I am attempting to add SVG circle elements by binding data to the current svg circle elements then calling enter.append As shown below:</p>
  
  <pre><code>var svg =d3.select("body").selectAll("svg");
  var circle = svg.selectAll("circle");
  var w=window.innerWidth;
  console.log(circle);
  circle.data([500,57,112,200,600,1000]);
  
  
  circle.enter().append("circle")
      .attr("cy",function(d) {
          return (d)
          })
      .attr("cx", function(i){
          return (i*100)
      })
      .attr("r", function(d) {
          return Math.sqrt(d);
          })
  </code></pre>
  
  <p>Which seems like it would add 3 new circle elements (because I already have 3 created). However, instead of getting these 3 new circle elements added, I am running into this error:</p>
  
  <blockquote>
    <p>Uncaught TypeError: Object [object SVGCircleElement],[objectSVGCircleElement],[object SVGCircleElement] has no method 'enter' </p>
  </blockquote>
  
  <p>I have done essentially the same thing with paragraphs, and it seems to work fine:</p>
  
  <pre><code>var p =d3.select("body").selectAll("p")
  .data([4, 8, 15, 16, 23, 42])
  .text(function(d) { return "I'm number " + d + "!"; });
  
   //Enter    
   p.enter().append("p")
  .text(function(d) { return "I'm number " + d + "!"; })
  .style("color", function(d, i) {
   return i % 2 ? "#000" : "#eee";
  });
  </code></pre>
  
  <p>However as soon as I try to add SVG Elements into it, I continue to get the same error.</p>
  
  <p>It seems like there should be just a syntax error or something, but I have gone through the code 5 trillion times, and can't find anything.</p>
  
  <p>Any help is much appreciated, and thank you before hand for your time.</p>
  
  <p>Isaac</p>