title: nvd3 line chart with string values on x-axis
body: <p>i'm new to nvd3 charts. i need a line chart, with string-values on the x-axis
  the chart should be like this <a href="http://i58.tinypic.com/25s7l9z.png">Bar Chart</a>, but i need a line, instead of bars</p>
  
  <p>my result looks like this <a href="http://i59.tinypic.com/29pribr.png">Line Chart</a>
  The values are all mapped to x=0</p>
  
  <p>my code</p>
  
  <pre><code>nv.addGraph(function() {
      var chart = nv.models.lineChart()
      .useInteractiveGuideline(true) 
      .transitionDuration(350)
      .x(function(d) { return d.x}) 
      .y(function(d) { return d.y}) 
      .showLegend(true)
      .showYAxis(true)
      .showXAxis(true);
  
      chart.xAxis.tickValues(["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]);
  
      d3.select(element + ' svg')
      .datum(data) 
      .call(chart);
  
      nv.utils.windowResize(chart.update);
  
      return chart;
  });
  </code></pre>
  
  <p>and my data</p>
  
  <pre><code>[{"color":"#a215af","key":"products","values":[
      {"label":"Monday","y":0,"x":"Monday"},
      {"label":"Tuesday","y":0,"x":"Tuesday"},
      {"label":"Wednesday","y":1,"x":"Wednesday"},
      {"label":"Thursday","y":6,"x":"Thursday"},
      {"label":"Friday","y":2,"x":"Friday"},
      {"label":"Saturday","y":0,"x":"Saturday"},
      {"label":"Sunday","y":13,"x":"Sunday"}]}] 
  </code></pre>
  
  <p>i tried a lot, but have no idea what to do.</p>
  
  <p>any help or suggestions would be great</p>
  
  <hr>
  
  <p><strong>Solution</strong>
  like dcclassics mentioned i took number values instead of strings
  and then used tickValues and tickFormat:</p>
  
  <pre><code>var days = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]
  
  chart.xAxis.tickValues([0, 1, 2, 3, 4, 5, 6])
  .tickFormat(function(d){
      return days[d]
  });
  </code></pre>