title: get data from selected slice of google charts PieChart
body: <p>i have big trouble trying to get the value of the selected slice of a PieChart when its clicked.</p>
  
  <p>The documentation says:</p>
  
  <blockquote>
    <p>selection_array: An array of selected objects, each one describing a
    data element in the underlying table used to create the visualization
    (a DataView or a DataTable). Each object has properties row and/or
    column, with the index of the row and/or column of the selected item
    in the underlying DataTable. If the row property is null, then the
    selection is a column; if the column property is null, then the
    selection is a row; <strong>if both are non-null, then it is a specific data</strong>
    *<em>item. You can call the DataTable.getValue()</em>* method to get the value of
    the selected item. The retrieved array can be passed into
    setSelection()</p>
  </blockquote>
  
  <p>in my case i get null from getSelection(), then i couldnt figure out what to do to get the value i want (the label of the column corresponding to that slice).</p>
  
  <p>Any answer will be very apreciated :)</p>
  
  <p>the example code of what im doing:</p>
  
  <pre><code>google.load('visualization', '1', {packages: ['controls']});
  google.setOnLoadCallback(drawVisualization);
  
  var data;
  var pie_area;
  
  function drawVisualization() {
    // Prepare the data
    data = google.visualization.arrayToDataTable([
      ["rbd", "nombre", "area", "dependencia", "simceMat", "ubicacionLon", "ubicacionLat", "simceLen", "nivel"],
      [22616, "Colegio Mozart", "Urbana", "Part_Sub", 228, -72.981148, -41.479872, 254, "Basico"],
      [22686,"Escuela Basica Camelias","Urbana","Muni",228,-72.980075,-41.474599,253, "Medio"],
      [40351,"Colegio Bosquemar","Urbana","Part_Sub",290,-72.981148,-41.479872,280, "Medio"],
      [7633,"Escuela Capitan Arturo Prat Chacon","Urbana","Muni",317,-72.909565,-41.474567,314, "Basico"],
      [7659,"Escuela Rural Alerce","Rural","Muni",230,-72.91767,-41.399121,249, "Basico"],
      [7671,"Escuela Rural Lagunitas","Rural","Muni",261,-72.964282,-41.459485,269, "Medio"],
      [7690,"Escuela Rural Rio Blanco","Rural","Muni",217,-72.638597,-41.455786,229, "Medio"],
      [7700,"Colegio San Francisco Javier","Urbana","Part",305,-72.942089,-41.470351,303, "Basico"],
      [7717,"Instituto Aleman de Puerto Montt","Urbana","Part",321,-72.932482,-41.470001,310, "Medio"],
      [7718,"The American School","Urbana","Part",317,-72.909,-41.456,314, "Medio"]
    ]);
  
    var fltArea = new google.visualization.ControlWrapper({
      'controlType': 'CategoryFilter',
      'containerId': 'f1',
      'options': {
        'filterColumnLabel': 'area',
        'ui': {
        'labelStacking': 'vertical',
          'allowTyping': false,
          'allowMultiple': false
        }
      }
    });
  
    pie_area = new google.visualization.ChartWrapper({
        'chartType': 'PieChart',
        'containerId': 'chart2',
        'options': {
          'width': 300,
          'height': 300,
          'legend': 'none',
          'title': 'Area',
          'pieSliceText': 'label'
        },
        'view': {'columns': [2]}
      });
  
    new google.visualization.Dashboard(document.getElementById('dashboard')).bind([fltArea], [pie_area]).draw(data);
    google.visualization.events.addListener(pie_area, 'select', onAreaSliceSelected);
  
  }
  
  function onAreaSliceSelected(){
    var sel = pie_area.getChart().getSelection(); //is always null
    console.log('you selected '+sel); //displays you selected null
  }
  </code></pre>