在任意值上方/下方绘制时，如何更改Chart.js线的颜色？

const threshold = 25;

new Chart('myChart', {
  type: 'line',
  plugins: [{
    afterLayout: chart => {
      let ctx = chart.chart.ctx;
      ctx.save();
      let yAxis = chart.scales["y-axis-0"];
      let yThreshold = yAxis.getPixelForValue(threshold);          
      let gradient = ctx.createLinearGradient(0, yAxis.top, 0, yAxis.bottom);   
      gradient.addColorStop(0, 'green'); 
      let offset = 1 / yAxis.bottom * yThreshold; 
      gradient.addColorStop(offset, 'green'); 
      gradient.addColorStop(offset, 'red'); 
      gradient.addColorStop(1, 'red');           
      chart.data.datasets[0].borderColor = gradient;
      ctx.restore();
    }
  }],
  data: {
    labels: ['A', 'B', 'C', 'D', 'E', 'F', 'G'],
    datasets: [{
      label: 'My Dataset',
      data: [32, 44, 29, 33, 18, 15, 30],
      fill: false
    }]
  },
  options: {
    legend: {
      display: false
    }
  }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.9.4/Chart.min.js"></script>
<canvas id="myChart" height="100"></canvas>