Best Of

I don't know about the default line chart. But I can do it in a brick.

<html>
<head>
  <script src="https://cdn.jsdelivr.net/npm/chart.js"></script>
  <script src="https://cdn.jsdelivr.net/npm/chartjs-plugin-annotation"></script>
</head>
<body>
  <canvas id="myChart"></canvas>
</body>
</html>

    const ctx = document.getElementById('myChart').getContext('2d');
    const myChart = new Chart(ctx, {
      type: 'line',
      data: {
        labels: ['January', 'February', 'March', 'April', 'May', 'June', 'July'],
        datasets: [{
          label: 'Dataset',
          data: [65, 59, 80, 81, 56, 55, 40],
          borderColor: 'rgba(75, 192, 192, 1)',
          borderWidth: 1,
          fill: false
        }]
      },
      options: {
        scales: {
          x: {
            beginAtZero: true
          },
          y: {
            beginAtZero: true
          }
        },
        plugins: {
          annotation: {
            annotations: {
              verticalLine: {
                type: 'line',
                xMin: 3,  // Position on the X axis
                xMax: 3,
                borderColor: 'rgba(255, 99, 132, 1)',
                borderWidth: 2
              },
              horizontalLine: {
                type: 'line',
                yMin: 70,  // Position on the Y axis
                yMax: 70,
                borderColor: 'rgba(54, 162, 235, 1)',
                borderWidth: 2
              }
            }
          }
        }
      }
    });