title: Callbacks on two-level doughnut chart in chartjs
body: <p>How to implement a proper callback on double-doughnut chart in chartjs?</p>
  
  <p>I know only the way to do something like this:</p>
  
  <pre><code>options: {
      tooltips: {
        callbacks: {
          label: function (tooltipItems, data) {
            return data.datasets[0].data[tooltipItems.index] + 'h';
          }
        }
      }
    }
  </code></pre>
  
  <p>But obviously it isn't working correctly. Because it returns data from first doughnut chart(<code>data.datasets[0].data</code>) and returns it for both charts.
  How to return data for both charts separately?</p>