title: Get value of column in dataset in javascript
body: <p>I would like to draw dynamic vertical marker line in my chart. Position of marker is available in dataset named <code>ml_data</code> in column <code>ml_position</code>. This is a dataset with one row and one column only. Is there a way how I can get value of <code>ml_position</code> in javascript? Currently I have the following working example:</p>
  
  <pre><code>function beforeDrawMarkerLine(axis, markerLine, icsc)
  {
      importPackage(Packages.org.eclipse.birt.chart.model.data.impl);
      importPackage(Packages.org.eclipse.birt.chart.model.component.impl);
      var ml_value = 20;
      markerLine.setValue(NumberDataElementImpl.create(ml_value)) ;
  }
  </code></pre>
  
  <p>The value is currently fixed (20). I would like to assign value of <code>ml_position</code> to <code>ml_value</code>.</p>