title: change format of X axis in c3.js chart
body: <p>I am generating a c3.js chart with the folling JSON and code, I'm trying to change the format of the X axis dates like this "November 2016" "N16". Can I get some help with this please? Thanks!</p>
  
  <pre><code>    response = [["November 2016","December 2016","January 2017","February 2017","March 2017","April 2017"],["total"
      ,2,43,60,51,46,110],["mammo",1,20,34,12,12,60],["face",1,20,16,30,32,32],["body",0,3,10,9,2,18],["photo"
      ,0,19,27,12,5,21],["scan",2,24,33,39,41,89]]
      initMonthlyUsageChart(response);
  
  function initMonthlyUsageChart(amounts) {
    var months = amounts.shift();
    c3.generate({
      bindto: '#monthly_usage_chart-js',
      data: { columns : amounts },
      axis: {
        x: {
          type: 'category',
          categories: months,
          tick: {
            rotate: 90,
            format: function (x) { // x comes in as a time string.
                  x[0] = x[0].map(function(date) {
                    return date.replace(/^(\w{1}).*(\d{2})$/gi, "$1$2");
                  });
              }
          }
        }
      }
    });
  }
  </code></pre>