title: Obtaining x-axis values in c3.js to customize the tooltip?
body: <p>I'm trying to edit the tooltip in a c3 line-chart.  Specifically, I need to access the current x-value within the <code>chart.tooltip.format.value</code> function.  However, the function is not passed the x-value explicitly. </p>
  
  <pre><code>var chart = c3.generate({
      tooltip: {
          format: {
              value: function (value, ratio, id, index) {
                  return value;
              }
          }
      },
  
      data: {
          x: 'YEAR',
          xFormat: '%Y',
          url: myURL',
      },
      axis: {
          x: {
              type: 'timeseries',
              tick: {
                  format: '%Y'
              }
          },
      },
  
  });
  </code></pre>