title: google charts html tooltip issue
body: <p>When I select one point at chart, tooltip html content is correct, but when select 2 and more points, tooltips merge in one and html of each one is visible. Are smbd know what i do wrong ? Can smbd help me ?   </p>
  
  <pre><code>var data = new google.visualization.DataTable();
    data.addColumn('date', 'Time');
  
    data.addColumn('number' , "id");
    data.addColumn({type: 'string', role: 'tooltip', p: {'html': true}});
  
    var dataArray = [];
  
    tooltipContent = "&lt;div class='google-chart-tooltip'&gt;&lt;center class='google-chart-tooltip-date'&gt;Date: " + date + "&lt;/center&gt;&lt;br /&gt;&lt;p&gt;Value1: " + value1 + ";&lt;br /&gt;Value2: " + value2 + ".&lt;/p&gt;&lt;/div&gt;";
  
    //fill dataArray
  
    data.addRows(dataArray);
  }
  var currentChartOptions = {
    tooltip: {
      isHtml: true,
      trigger: 'selection'
    },
    selectionMode: 'multiple',
    interpolateNulls: true,
    hAxis: {
      title: 'Time'
    },
    vAxis: {
        title: "Values"
    }
  };
  </code></pre>
  
  <p>Follows an image of the issue:</p>
  
  <p><a href="https://i.stack.imgur.com/cuJ5q.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/cuJ5q.jpg" alt="Issue Picture"></a></p>