title: D3: Cannot select an item based on its id
body: <p>I would like to visualize a bunch of circles on my screen and make them dragable. 
  I am assigning an unique id to each circle. Later I am trying to select a particular circle by its id inside the <strong>on("dragend", ..)</strong> event but I am receiving following error: </p>
  
  <p>SyntaxError: An invalid or illegal string was specified1 d3.js:549:0</p>
  
  <pre><code>    var drag = d3.behavior.drag()
          .on("drag", function(d,i) {
              d3.select(this).attr("cx", d3.event.x)
              d3.select(this).attr("cy", d3.event.y) 
           })
  
          .on("dragend",function(d,i){    
              var previous= d3.select("#3")  // Here I am getting Error
              console.log(previous);
           });
  
  var circle = svg.selectAll('circle')
      .data(csv)
      .enter()
      .append("circle")
      .attr("stroke", "black")
      .attr("id",function(d,i){return i;})
      .attr("fill", function(d) {return color(d.Cyl);})
      .attr("cx", function(d) { return xScale(d.Weight); })
      .attr("cy", function(d) { return yScale(d.DealerCost); })
      .attr("r", function(d) { return d.EngineSize+2; })
       .call(drag);
  </code></pre>
  
  <p>How can I solve this problem?</p>