title: convert date to unix time stamp in nvd3 x-axis
body: <p>Here I'm getting date as 1970-01-01 by default.Can anyone please help me out in converting "2015-10-01" to timestamp.</p>
  
  <p>My date : </p>
  
  <pre><code>"date": "2015-10-01"
  
  
  xAxis: {
                  axisLabel: 'Months',
                  ticks: d3.time.months,
                  tickFormat: function (d) {                    
  
  
  
                     return  d3.time.format('%Y %b')(new Date(d));
  
                  }
  </code></pre>
  
  <p>I did this thing,which solved my issue</p>
  
  <pre><code>x: function (d) { return  d3.time.format('%Y-%m-%d').parse(d.date); },
              y: function (d) { return d.trendingValue; },
             useInteractiveGuideline: true,
             xScale: d3.time.scale(),
              xAxis: {
                  axisLabel: 'Months',
                  ticks: d3.time.months,
                  tickFormat: function (d) {                    
  
                     return  d3.time.format('%Y-%m-%d')(new Date(d));
  
                  }
              }
  </code></pre>