title: Trying to get _index of clicked bar in ChartJS
body: <p>I have a bar chart in vue-charts where when clicked I want to display the background data in a table. This is the render</p>
  
  <pre><code>startPie: function(canvas, type){
                  let chart = new Chart(canvas, {
                      type: type,
                      data: {
                          labels: this.labels,
                          datasets: this.chart_data
                      },
                      options: {
                          responsive: true,
                          animation:{ animateScale:true },
                          title: { display: false },
                          legend: { display: false },
                          scales: {
                              yAxes: [{
                                  gridLines: {
                                      color: "#989898"
                                  },
                                  ticks: {
                                      fontColor: "#989898",
                                      fontSize: 16,
                                      beginAtZero: true,
                                      callback: function(value) {
                                          let ranges = [
                                              { divider: 1e6, suffix: 'M' },
                                              { divider: 1e3, suffix: 'k' }
                                          ];
                                          function formatNumber(n) {
                                              for (let i = 0; i &lt; ranges.length; i++) {
                                                  if (n &gt;= ranges[i].divider) {
                                                      return (n / ranges[i].divider).toString() + ranges[i].suffix;
                                                  }
                                              }
                                              return n;
                                          }
                                          return '$' + formatNumber(value);
                                      }
                                  }
                              }],
                              xAxes: [{
                                  ticks: {
                                      fontColor: "#989898",
                                      fontSize: 12
                                  }
                              }]
                          },
                          plugins: {
                              datalabels: {
                                  anchor: 'center',
                                  font: {
                                      size: 16,
                                      weight: 'bold'
                                  },
                                  formatter: function(value, context) {
                                      return '$' + Number(value).toLocaleString();
                                  },
                                  color: 'white'
                              }
                          },
                          'onClick':(evt, item) =&gt; {
                              console.log('CLICK', item[0]);
                          }
                      }
                  })
              }
  </code></pre>
  
  <p>In the onClick Event in the console I can see the index</p>
  
  <blockquote>
    <p>CLICK ChartElement {_chart: Chart, _datasetIndex: 0, _index: 4,
    hidden: false, _xScale: ChartElement, …}</p>
  </blockquote>
  
  <p>I can't access it though without an error. I have tried
  <code>item[0]._index</code>, <code>item[0]['_index']</code>, <code>item[0]['ChartElement']._index</code>, <code>item[0]['ChartElement']['_index']</code> </p>
  
  <p>Any help is appreciated.</p>