如何获取Google数据图表选择处理程序的列索引

Question

我使用以下示例将Google数据图表与Selection处理程序一起使用。但它表示无法返回Column索引。 它说它应该 任何人都可以给我一些关于如何实现处理程序来选择列索引的提示？

// Note: This sample shows the select event.
// The select event is a generic select event,
// for selecting rows, columns, and cells.
// However, in this example, only rows are selected.
// Read more here: http://code.google.com/apis/visualization/documentation/gallery/table.html#Events

function drawVisualization() {
  visualization = new google.visualization.Table(document.getElementById('table'));
  visualization.draw(data, null);

  // Add our selection handler.
  google.visualization.events.addListener(visualization, 'select', selectHandler);
}

// The selection handler.
// Loop through all items in the selection and concatenate
// a single message from all of them.
function selectHandler() {
  var selection = visualization.getSelection();
  var message = '';
  for (var i = 0; i < selection.length; i++) {
    var item = selection[i];
    if (item.row != null && item.column != null) {
      var str = data.getFormattedValue(item.row, item.column);
      message += '{row:' + item.row + ',column:' + item.column + '} = ' + str + '\n';
    } else if (item.row != null) {
      var str = data.getFormattedValue(item.row, 0);
      message += '{row:' + item.row + ', (no column, showing first)} = ' + str + '\n';
    } else if (item.column != null) {
      var str = data.getFormattedValue(0, item.column);
      message += '{(no row, showing first), column:' + item.column + '} = ' + str + '\n';
    }
  }
  if (message == '') {
    message = 'nothing';
  }
  alert('You selected ' + message);
}