Tooltip customization not working for Google Charts
<p>I'm trying to create custom tooltips for a Google Charts timeline chart, but I can't get my custom string to show up. The chart still displays the default tooltip.</p>

<pre><code>  google.charts.load('current', {'packages':['timeline']});
google.charts.setOnLoadCallback(drawChart);

function drawChart() {
  var data = new google.visualization.DataTable();
  data.addColumn('string', 'Team');
  data.addColumn('date', 'Season Start Date');
  data.addColumn('date', 'Season End Date');
  data.addColumn({type: 'string', role: 'tooltip'});

  data.addRows([
    ['Baltimore Ravens',     new Date(2000, 8, 5), new Date(2001, 1, 5), 'my tooltip'],
    ['New England Patriots', new Date(2001, 8, 5), new Date(2002, 1, 5), 'my tooltip'],
  ]);

  var options = {
    height: 450,
    timeline: {
      groupByRowLabel: true
    }
  };

  var chart = new google.visualization.Timeline(document.getElementById('chart_div'));

  chart.draw(data, options);
}
</code></pre>

<p>Here is the Google Charts documentation: <a href="https://developers.google.com/chart/interactive/docs/customizing_tooltip_content" rel="nofollow">https://developers.google.com/chart/interactive/docs/customizing_tooltip_content</a></p>

<p>JSFiddle: <a href="https://jsfiddle.net/nt12ev9h/" rel="nofollow">https://jsfiddle.net/nt12ev9h/</a></p>

<p>What am I doing wrong?</p>