如何在图表 js 中绘制一条线到最高数据点？

Question

我一直在使用 Chart JS 文档，但我认为这可能更多是基于问题的计算机科学/数学。我正在尝试绘制一条从图表底部延伸到最高数据点顶部的线。这是一个工作代码示例的链接：https://stackblitz.com/edit/react-pvzbwc

这个想法是让图表看起来像这样，点正好在数据的顶部结束：https://i.stack.imgur.com/L8d0H.jpg

以下是我的 after draw 钩子：

// draw a line when someone hovers over a data point
afterDatasetDraw: (chart) => {
          // console.log(chart)
          if (chart.tooltip._active && chart.tooltip._active.length) {
            const activePoint = chart.tooltip._active[0];
            console.log(activePoint.tooltipPosition())
            const ctx = chart.ctx;
            const y_axis = chart.scales['y-axis-0'];
            const x = activePoint.tooltipPosition().x;
            const yData = activePoint._chart.config.data.datasets[activePoint._datasetIndex].data[activePoint._index].y;

            const topY = y_axis.top;
            const bottomY = y_axis.bottom;
            // draw line
            ctx.save();
            ctx.beginPath();
            ctx.moveTo(x, topY);
            ctx.lineTo(x, bottomY);
            ctx.lineWidth = 2;
            ctx.strokeStyle = '#000';
            ctx.stroke();
            ctx.restore();
         }
        }

当我需要在顶部画一条直线时效果很好，但当我只想在最高数据点的顶部看到它时就不行了。我注意到 topY 是一个静态不变的数字。我想知道是否有一种方法可以根据图表的笛卡尔点计算顶部位置？

绝对赞赏任何见解。

Answer 1

经过一番挣扎，我已经回答了这个问题。这比我提出的问题要简单得多。事实证明，我只需要计算数据点与图形之间的比率（以像素为单位），然后在该点处实现一条线。

我已经使用 API 将其放入 plugin：

/// default values
lineHeightAnnotation: {
  // defaults to have line to the highest data point on every tick
  always: true,
  // optionally, only have line draw to the highest datapoint nearest the user's hover position
  hover: false,
  // colors of the line
  color: '#000',
  // name of yAxis
  yAxis: 'y-axis-0',
  // weight of the line
  lineWeight: 1.5,
   /// sets shadow for ALL lines on the canvas
  shadow: {
    // color of the shadow
    color: 'rgba(0,0,0,0.35)',
    // blur of the shadow
    blur: 10,
    /// shadow offset
    offset: {
      // x offset
      x: 0,
      // y offset
      y: 3
    }
  },
  // dash defaults at [10, 10]
  noDash: true,
}

逻辑：

/**
 * Vars
 * maxY - the tallest data point on the graph
 * tickMax - the tallest tick on the y axis
 * bottomY - the lowest point of the graph
 * additionalOffsets = dataset.borderWidth * 2
 * 
 *                               bottomY * maxY      
 * highestDataY =   bottomY -  -------------------   + additionOffsets
 *                                   tickMax
 */

功能：

   afterDatasetDraw: (chart) => {
      // draw a dashed line when someone hovers over a data point
      if (chart.tooltip._active && chart.tooltip._active.length) {
        const activePoint = chart.tooltip._active[0];
        const ctx = chart.ctx;

        const x = activePoint.tooltipPosition().x;
        const yAxis = chart.scales['y-axis-0'];

        const tickMax = yAxis.ticksAsNumbers[0] // first index is always the tallest
        const tickLow = yAxis.ticksAsNumbers[yAxis.ticksAsNumbers.length - 1]
        const topY = yAxis.top; // clientRect.top + chart.padding.y
        const bottomY = yAxis.bottom; // clientRect.bottom

        let maxY = 1;
        let borderWidth = 0;
        const datasets = chart.config.data.datasets
        datasets.forEach((set, i) => {
          // get maximum Y value
          // get borderWidth of that dataset
          let point = set.data[activePoint._index].y
          if(point > maxY) {
            maxY = parseInt(point, 10) - parseInt(set.borderWidth, 10)
            borderWidth = parseInt(set.borderWidth, 10)
          }
        });

        let yBRatio = bottomY * (maxY - tickLow)
        let tMRatio = yBRatio / (tickMax - tickLow)
        let highestDataY = bottomY - tMRatio + (borderWidth * 2)

        // draw line
        ctx.save();
        ctx.beginPath();
        ctx.setLineDash([10, 10]);
        ctx.moveTo(x, highestDataY);
        ctx.lineTo(x, bottomY);
        ctx.lineWidth = 1.5;
        ctx.strokeStyle = '#000';
        ctx.stroke();
        ctx.restore();
      }