title: nvd3: set background fill behind line chart
body: <p>I have some data which is used in one place to make a heatmap, and in another place to make a line chart.  I want to put a gradient behind the line chart so that points on the line chart appear in front of the color used in the heatmap.</p>
  
  <p>If I understand right, all that's needed is to set the fill on the appropriate svg element, but I haven't found any indication that nvd3 will help me find that element.  Can I do this with the nvd3 API, or do I have to reverse-engineer the chart to find or insert that element?</p>