title: How to change tooltip label according to number in c3 chart?
body: <p>I want to display tooltip label as a text in place of number, I have fix six string like A = 1, B = 2, C = 3, D = 4, E = 5, E = 6. </p>
  
  <p>I am using number because i can't plot the graph with text string so i am using number from 1 to 6. Please help me. I am using c3 chart in laravel5.</p>
  
  <p>Here is the code:</p>
  
  <pre><code>c3.generate({
          bindto: '#activity-user-grade-chart',
          size: {
              height: 240,
              width: 480
          },
          data: {
              x: 'x',
              xFormat: '%d-%m', // how the date is parsed
              type: 'bar',
              columns: [
                  ['01-08','01-09','01-10','01-11'],
                  [1,4,3,5]
              ]
          },
          color: {
              pattern: ['#FEBE29']
          },
          axis: {
              x: {
                  type: 'timeseries',
                  tick: {
                      format: '%d-%m' // how the date is displayed
                  }
              }
          }
      }); 
  </code></pre>
  
  <p>Expected tooltip :    </p>
  
  <p><a href="https://i.stack.imgur.com/tktB6.gif" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/tktB6.gif" alt="enter image description here"></a></p>
  
  <pre><code>I want like this **Grade | A** in place of numeric value.
  </code></pre>