title: Bar chart columns not grouping in D3/C3
body: <p>I am using D3 (technically C3) to try to create a grouped bar chart.</p>
  
  <p>The chart I am trying to make would be a bar chart of Donations per occupation, grouped by PaymentType. For the life of me I can't figure out how to do the grouping.</p>
  
  <p>This is a code I have so far. It produces everything I want, except the grouping by PaymentType:</p>
  
  <pre><code>var x_values = ["Occupation", "DENTIST", "PROGRAMMER/ANALYST", "CORPORATE SECRETARY", "PHYSICIAN", "PROFESSOR", "PRESIDENT", "PROFESSOR", "MANAGER", "CORPORATE SECRETARY", "PRESIDENT", "PHYSICIAN", "DENTIST", "PROGRAMMER/ANALYST", "CO-OWNER", "MANAGER", "CO-OWNER"]
  var y_values = ["Donation", 11340, 12335, 3259, 18041, 10415, 35900, 14660, 21985, 2950, 15110, 16705, 370, 22255, 13055, 4000, 20740]
  var z_values = ["PaymentType", "MONETARY", "NON-MONETARY", "NON-MONETARY", "MONETARY", "MONETARY", "NON-MONETARY", "NON-MONETARY", "MONETARY", "MONETARY", "MONETARY", "NON-MONETARY", "NON-MONETARY", "MONETARY", "MONETARY", "NON-MONETARY", "NON-MONETARY"]
  
  var chart = c3.generate({
      data: {columns: [y_values],
              type: 'bar',
              groups: [z_values],},
      axis: {x: 
              {type: 'category',
              categories: x_values}}
  });
  </code></pre>