更改饼图切片Google Chart上的文字方向

Question

我正在使用Google Chart api在网页上显示数据。我想在饼图切片上更改文本方向。我的图表代码如下： -

<script type="text/javascript" src="https://www.google.com/jsapi"></script>
<script type="text/javascript">
  google.load("visualization", "1", {packages:["corechart"]});
  google.setOnLoadCallback(drawChart);
  function drawChart() {
    var data = google.visualization.arrayToDataTable([
      ['Language', 'Speakers (in millions)'],
      ['Assamese', 13], ['Bengali', 83], ['Bodo', 1.4],
      ['Dogri', 2.3], ['Gujarati', 46], ['Hindi', 300],
      ['Kannada', 38], ['Kashmiri', 5.5], ['Konkani', 5],
      ['Maithili', 20], ['Malayalam', 33], ['Manipuri', 1.5],
      ['Marathi', 72], ['Nepali', 2.9], ['Oriya', 33],
      ['Punjabi', 29], ['Sanskrit', 0.01], ['Santhali', 6.5],
      ['Sindhi', 2.5], ['Tamil', 61], ['Telugu', 74], ['Urdu', 52]
    ]);

    var options = {
      title: 'Indian Language Use',
      legend: 'none',
      pieSliceText: 'label',
      pieSliceTextStyle: {color: 'black'},
      chartArea:{left:7,top:7,width:'94%',height:'94%'} 
    };
    var chart = new google.visualization.PieChart(document.getElementById('piechart'));
    chart.draw(data, options);
  }
</script>

所需图表： -