title: unable to horizontal draw reference line nvd3 bubble chart
body: <p>I am trying to draw a horizontal line (for reference) in the nvd3 bubble chart. I tried to append a line to svg chart</p>
  
  <pre><code>.append('line')
      .attr({
          x1: x(0),
          y1: 10,
          x2: x(3),
          y2: 10
  });
  </code></pre>
  
  <p>But am not successful. <a href="http://jsfiddle.net/dm89/NYJpU/8/" rel="nofollow">Here</a> is the fiddle. Note that the <code>y1</code> and <code>y2</code> values are same (i.e., 10) because I am trying to draw a horizontal line ( <code>y=10</code> ).</p>