title: ChartJS v2 custom tooltip for rLabel
body: <p>I have a ChartJS (v2) bubble chart with three dimensions: x, y, and r (radius of the bubble).</p>
  
  <p>Following <a href="https://stackoverflow.com/questions/37158477/where-put-multitooltiptemplate-in-chat-js-v2-x">this answer</a>, I have this code for customizing the tooltip:</p>
  
  <pre><code>tooltips: {
    callbacks: {
      label: function (tooltipItem, data) {
        var datasetLabel = data.datasets[tooltipItem.datasetIndex].label || '';
        return datasetLabel + ' : ' + tooltipItem.rLabel + '% has price of ' + tooltipItem.yLabel + ' USD';
      }
    }
  }
  </code></pre>
  
  <p>It almost works except that <code>tooltipItem.rLabel</code> is displayed as <code>undefined</code>. If I input <code>tooltipItem.xLabel</code> the tooltip displays correctly with the value of <code>x</code>. However, I want to display the value of <code>r</code>.</p>
  
  <p>Does anyone know a solution for this?</p>