title: How to add 'tooltip' to Timeline chart in angular-google-charts?
body: <p>I'm trying to add tooltip to Timeline chart using angular-google-charts library in Angular 7 application. When I add a fifth column at index = 2, angular-google-charts throws as error saying "Expecting 4 columns not 5"</p>
  
  <p>When I remove the tooltip column providing only 4 columns (Group, Row, Start, End) the Timeline visualisation works fine. However as soon as I add the Tooltip column it throws error saying expecting 4 columns but received 5.</p>
  
  <h2>component.ts</h2>
  
  <pre><code>type: string = 'Timeline'
  chartData: Array&lt;Array&lt;any&gt;&gt; = [
    ['Group1', 'Row1', 'Tooltip1', new Date(2019, 01, 01), new Date(2019, 02, 01)],
    ['Group2', 'Row2', 'Tooltip2', new Date(2019, 03, 01), new Date(2019, 04, 01)]
  ]
  colNames: Array&lt;any&gt; = ["Group", "Row", "Tooltip", "Start", "End"] 
  colRoles: Array&lt;any&gt; = [
    { role: 'tooltip', type: 'string', index: 2, p: {html: true} },
  ]
  options: {} = { tooltip: {isHtml: true} }
  </code></pre>
  
  <h2>component.html</h2>
  
  <pre class="lang-html prettyprint-override"><code>&lt;google-chart 
      [type]="type"
      [data]="chartData" 
      [options]="options" 
      [columnNames]="colNames"
      [roles]="colRoles"&gt;
  &lt;/google-chart&gt;
  
  </code></pre>
  
  <h2>app.module.ts</h2>
  
  <pre><code>import { GoogleChartsModule } from 'angular-google-charts';
  @NgModule({
  ...
  imports: [
    GoogleChartsModule
  ],
  </code></pre>
  
  <p>I expect the tooltip to show the additional text I am putting in the column index 2. Please help me sort this out.</p>