ChartJS: y轴正负部分的两个标题

var chartData = {
  labels: ['January', 'February', 'March', 'April', 'May', 'June', 'July'],
  datasets: [{
    label: 'Positive',
    data: [10, 20, 30, 40, 50, 60, 70],
    backgroundColor: 'rgba(75, 192, 192, 0.2)',
    borderColor: 'rgba(75, 192, 192, 1)',
    borderWidth: 1
  }, {
    label: 'Negative',
    data: [-10, -20, -30, -40, -50, -60, -70],
    backgroundColor: 'rgba(255, 99, 132, 0.2)',
    borderColor: 'rgba(255, 99, 132, 1)',
    borderWidth: 1
  }]
};

var chartOptions = {
  scales: {
    yAxes: [{
      scaleLabel: {
        display: true,
        labelString: 'Positive'
      },
      ticks: {
        beginAtZero: true
      }
    }, {
      scaleLabel: {
        display: true,
        labelString: 'Negative'
      },
      ticks: {
        beginAtZero: true
      }
    }]
  }
};

var ctx = document.getElementById('myChart').getContext('2d');
var myChart = new Chart(ctx, {
  type: 'bar',
  data: chartData,
  options: chartOptions
});