title: nvd3.js line chart -- add vertical lines
body: <p>I need to add <em>several</em> vertical lines (say 10 or 20) to an nvd3 line chart.</p>
  
  <p>The question <a href="https://stackoverflow.com/questions/19582005/nvd3-line-chart-with-vertical-line">here</a> suggests adding a series for this, but I would need to add 20 series, overcrowding the legend and the interactive tooltip.</p>
  
  <p>From what I understand this can't be done out-of-the-box (<em>please</em> correct me if I'm wrong), so my question is what is the easiest way of doing this:</p>
  
  <ol>
  <li>Add D3 lines to the DOM (how would I go about scale, positioning them horizontally, etc?)</li>
  <li>Add generic support for this in nvd3</li>
  <li>Add support for hiding specific series from the legend and from the tooltip, and add 20 series</li>
  <li>Any other idea?</li>
  </ol>