Display item in legend even if value = 0 with Google Charts Tools Pie Chart

setting sliceVisibilityThreshold as zero will solve your problem.

function drawVisualization() {
  // Create and populate the data table.
  var data = google.visualization.arrayToDataTable([
    ['Task', 'Hours per Day'],
    ['Work', 11],
    ['Eat', 0],
    ['Commute', 2],
    ['Watch TV', 2],
    ['Sleep', 7]
  ]);

  // Create and draw the visualization.
  new google.visualization.PieChart(document.getElementById('visualization')).
      draw(data, {title:"So, how was your day?",
                 sliceVisibilityThreshold:0
                 });
}
​

I was recently adding the Google Charts and was facing problem in it, for Adding zero value in it.
Thanks for @ocanal, I used sliceVisibilityThreshold:0, but in some other way.

<script type="text/javascript">
        google.charts.load('current', {'packages':['corechart']});
      google.charts.setOnLoadCallback(drawChart);

      function drawChart() {

        var data = google.visualization.arrayToDataTable([
          ['Task', 'Hours per Day'],
          ['B-TRIPS',     <?php echo $arr_get_a_org['total_trips']; ?>],
          ['Reimbursed',      <?php echo $arr_get_a_org['reimbursed_trips']; ?>],
          ['Approved',  <?php echo $arr_get_a_org['approved_trips']; ?>],
          ['Pending', <?php echo $arr_get_a_org['pending_trips']; ?>]
         // ['Sleep',    <?php echo $arr_get_a_org['total_trips']; ?>]
        ]);

        var options = {
          title: 'OVERVIEW',
          backgroundColor:'#e2e1e0',
          pieSliceText:'value',
          sliceVisibilityThreshold :0

        };

        var chart = new google.visualization.PieChart(document.getElementById('piechart'));

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

As the way of defining of options have changed, for more info check out Google Chart site