title: Position tooltip in center of bar
body: <p>Can't find a way to position tooltip in center of bar like this (yeah, I know it is not exactly in center on this screenshot, but still):</p>
  
  <p><a href="https://i.stack.imgur.com/AEbnL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/AEbnL.png" alt="enter image description here"></a></p>
  
  <p>If I use <a href="http://www.chartjs.org/docs/latest/configuration/tooltip.html#tooltip-model" rel="nofollow noreferrer"><code>custom</code></a> tooltip option, I can get only <code>x</code> and <code>y</code> positions of a caret on top of a bar. But can't get height/width of a bar.</p>
  
  <p>Here is a part of an options object that I pass to Chart constructor:</p>
  
  <pre><code>const options = {
    tooltips: {
      enabled: false,
      custom: (tooltip) =&gt; {
        // Retrieving valuable props from tooltip (caretX, caretY)
        // and creating custom tooltip that is positioned
        // on top of a bar
      }
    }
    // other options
  }
  
  const chart = new Chart(ctx, {
    type: 'bar',
    data,
    options
  })
  </code></pre>