Draw horizontal line on chart in chart.js on v2

You could use the chart.js annotation plugin to easily draw lines on your chart without having to mess with rendering pixels in your canvas manually (the old approach that is giving you errors). Note, the plugin is created/supported by the same team as chart.js and is mentioned in the chart.js docs.

Here is an example codepen demonstrating creating a line on a chart.

Once you add the plugin, you simply just set annotation properties in your chart config. Here is an example.

var myChart = new Chart(ctx, {
  type: 'line',
  data: {
    labels: ["January", "February"],
    datasets: [{
      label: 'Dataset 1',
      borderColor: window.chartColors.blue,
      borderWidth: 2,
      fill: false,
      data: [2, 10]
    }]
  },
  options: {
    responsive: true,
    title: {
      display: true,
      text: 'Chart.js Draw Line On Chart'
    },
    tooltips: {
      mode: 'index',
      intersect: true
    },
    annotation: {
      annotations: [{
        type: 'line',
        mode: 'horizontal',
        scaleID: 'y-axis-0',
        value: 5,
        borderColor: 'rgb(75, 192, 192)',
        borderWidth: 4,
        label: {
          enabled: false,
          content: 'Test label'
        }
      }]
    }
  }
});