Angular-Chart-JS - 根据点的范围具有不同填充颜色的折线图

First, we must extend Chart.elements.Line and overwrite it's draw() method so that we can build the linear gradient based upon the position of each point, use it as the line fill, and then draw the line.

// save the original line element so we can still call it's 
// draw method after we build the linear gradient
var origLineElement = Chart.elements.Line;

// define a new line draw method so that we can build a linear gradient
// based on the position of each point
Chart.elements.Line = Chart.Element.extend({
  draw: function() {
    var vm = this._view;
    var backgroundColors = this._chart.controller.data.datasets[this._datasetIndex].backgroundColor;
    var points = this._children;
    var ctx = this._chart.ctx;
    var minX = points[0]._model.x;
    var maxX = points[points.length - 1]._model.x;
    var linearGradient = ctx.createLinearGradient(minX, 0, maxX, 0);

    // iterate over each point to build the gradient
    points.forEach(function(point, i) {
      // `addColorStop` expects a number between 0 and 1, so we
      // have to normalize the x position of each point between 0 and 1
      // and round to make sure the positioning isn't too percise 
      // (otherwise it won't line up with the point position)
      var colorStopPosition = roundNumber((point._model.x - minX) / (maxX - minX), 2);

      // special case for the first color stop
      if (i === 0) {
        linearGradient.addColorStop(0, backgroundColors[i]);
      } else {
        // only add a color stop if the color is different
        if (backgroundColors[i] !== backgroundColors[i-1]) {
          // add a color stop for the prev color and for the new color at the same location
          // this gives a solid color gradient instead of a gradient that fades to the next color
          linearGradient.addColorStop(colorStopPosition, backgroundColors[i - 1]);
          linearGradient.addColorStop(colorStopPosition, backgroundColors[i]);
        }
      }
    });

    // save the linear gradient in background color property
    // since this is what is used for ctx.fillStyle when the fill is rendered
    vm.backgroundColor = linearGradient;

    // now draw the lines (using the original draw method)
    origLineElement.prototype.draw.apply(this);
  }               
});

那么，我们还要扩展折线图，保证图表使用的线元素就是我们上面扩展的那个(因为这个属性在加载时已经设置好了)

// we have to overwrite the datasetElementType property in the line controller
// because it is set before we can extend the line element (this ensures that 
// the line element used by the chart is the one that we extended above)
Chart.controllers.line = Chart.controllers.line.extend({
  datasetElementType: Chart.elements.Line,
});