title: react-chartjs-2 tooltip callback not working
body: <p>I'm using react-chartjs-2 library to make simple charts in React. I tried to customize a bit the tooltip by adding a title:</p>
  <pre><code>tooltips: {
      callbacks: {
        label: (tooltipItem, data) =&gt; {
          return tooltipItem?.value + ' test';
        }
      }
    }
  </code></pre>
  <p>The code Sandbox: <a href="https://codesandbox.io/s/zealous-mestorf-ste8u" rel="nofollow noreferrer">https://codesandbox.io/s/zealous-mestorf-ste8u</a><br>
  The code doesn't work although I followed the chart.js example and also many other custom tooltip example (i.e answer from this question: <a href="https://stackoverflow.com/questions/48789218/react-chartjs-2-pie-chart-tooltip-percentage">React-chartjs-2: Pie Chart tooltip percentage</a>
  ).</p>