title: How to add a title for a NVD3.js graph
body: <p>I want to add a title text over the graph in NVD3.js.</p>
  
  <p>I tried the following,</p>
  
  <pre><code>nv.addGraph(function() {
  var chart = nv.models.linePlusBarWithFocusChart()
      .margin({top: 30, right: 60, bottom: 50, left: 70})
      .x(function(d,i) { return i })
      .color(d3.scale.category10().range());
  
  chart.append("text")
      .attr("x", 200)             
      .attr("y", 100)
      .attr("text-anchor", "middle")  
      .text("Sample Charts");
  }
  </code></pre>
  
  <p>Its (Highlighted) working for D3.js but not working for NVD3.js. I'm getting a Blank page..</p>
  
  <p>I want to place the text over the Chart as like the following image (Which is created in Paint)</p>
  
  <p><img src="https://i.stack.imgur.com/ELEuR.png" alt="enter image description here"></p>
  
  <p>How can I achieve this?</p>