javascript - 如何获取 Google 数据图表选择处理程序的列索引

Question

我采用以下示例将 google 数据图表与选择处理程序一起使用。但它说它无法返回列索引。它说应该有人能给我一些关于如何实现处理程序来选择列索引的提示吗？

// 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);
}