Outside of chart

const config = { type: 'line', data, options: { layout: { padding: { right: 50 } }, scales: { y: { stacked: true } }, plugins: { annotation: { clip: false, annotations: { annotation } } } } };

const config = {
  type: 'line',
  data,
  options: {
    layout: {
      padding: {
        right: 50
      }
    },
    scales: {
      y: {
        stacked: true
      }
    },
    plugins: {
      annotation: {
        clip: false,
        annotations: {
          annotation
        }
      }
    }
  }
};

const annotation = { type: 'line', borderColor: 'black', borderWidth: 3, scaleID: 'y', value: 55, beforeDraw: drawExtraLine };

const annotation = {
  type: 'line',
  borderColor: 'black',
  borderWidth: 3,
  scaleID: 'y',
  value: 55,
  beforeDraw: drawExtraLine
};

const DATA_COUNT = 8; const MIN = 10; const MAX = 100; Utils.srand(8); const labels = []; for (let i = 0; i < DATA_COUNT; ++i) { labels.push('' + i); } const numberCfg = {count: DATA_COUNT, min: MIN, max: MAX}; const data = { labels: labels, datasets: [{ data: Utils.numbers(numberCfg) }] };

const DATA_COUNT = 8;
const MIN = 10;
const MAX = 100;
Utils.srand(8);
const labels = [];
for (let i = 0; i < DATA_COUNT; ++i) {
  labels.push('' + i);
}
const numberCfg = {count: DATA_COUNT, min: MIN, max: MAX};
const data = {
  labels: labels,
  datasets: [{
    data: Utils.numbers(numberCfg)
  }]
};

function drawExtraLine(context) { const ctx = context.chart.ctx; const width = context.chart.canvas.width; const {x, y, x2, y2, options} = context.element; ctx.save(); ctx.lineWidth = options.borderWidth; ctx.strokeStyle = options.borderColor; ctx.setLineDash([6, 6]); ctx.lineDashOffset = options.borderDashOffset; ctx.beginPath(); ctx.moveTo(0, y); ctx.lineTo(x, y); ctx.moveTo(x2, y2); ctx.lineTo(width, y); ctx.stroke(); ctx.restore(); return true; }

function drawExtraLine(context) {
  const ctx = context.chart.ctx;
  const width = context.chart.canvas.width;
  const {x, y, x2, y2, options} = context.element;
  ctx.save();
  ctx.lineWidth = options.borderWidth;
  ctx.strokeStyle = options.borderColor;
  ctx.setLineDash([6, 6]);
  ctx.lineDashOffset = options.borderDashOffset;
  ctx.beginPath();
  ctx.moveTo(0, y);
  ctx.lineTo(x, y);
  ctx.moveTo(x2, y2);
  ctx.lineTo(width, y);
  ctx.stroke();
  ctx.restore();
  return true;
}