Chartjs-How do you insert a linear horizontal line in chart.js

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you should update data source and update chart.

addline(25);
function addline(num){
   var item={
        order: myLineChart.data.datasets.length-1,
        label: 'Title',
        type: 'line',
        borderColor: window.chartColors.black,
        data:new Array(myLineChart.data.datasets[0].data.length).fill(num)
   }
   myLineChart.data.datasets.push(item)
   myLineChart.update();
};

var canvas = document.getElementById('bmi-chart');
window.chartColors = {
  red: 'rgb(255, 99, 132)',
  orange: 'rgb(255, 159, 64)',
  yellow: 'rgb(255, 205, 86)',
  green: 'rgb(51, 204, 51)',
  blue: 'rgb(54, 162, 235)'
};

var myLineChart = new Chart(canvas, {
  type: 'line',
  data: {
    labels: ['2020-05-20', '2020-05-21', '2020-05-23', '2020-05-24', '2020-05-25', '2020-06-02', '2020-06-03', '2020-06-04', '2020-06-05', '2020-06-06', '2020-06-07'],
    datasets: [{
        order: 1,
        label: 'Current BMI',
        fill: false,
        borderColor: window.chartColors.blue,
        data: [36.0, 24.0, 29.0, 26.0, 23.0, 26.0, 26.0, 26.0, 26.0, 26.0, 26.0]
      }, {
        order: 2,
        label: 'Desired BMI',
        type: 'line',
        borderColor: window.chartColors.red,
        data: [24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24]
      }

    ]
  },
  options: {
    responsive: true,
    backgroundRules: [{
        backgroundColor: window.chartColors.blue,
        yAxisSegement: 18.5
      },
      {
        backgroundColor: window.chartColors.green,
        yAxisSegement: 24.9
      },
      {
        backgroundColor: window.chartColors.yellow,
        yAxisSegement: 29.9
      },
      {
        backgroundColor: window.chartColors.orange,
        yAxisSegement: 34.9
      }, {
        backgroundColor: window.chartColors.red,
        yAxisSegement: Infinity
      }
    ],
    scales: {
      yAxes: [{
        ticks: {
          beginAtZero: false,
          stepSize: 1
        }
      }],
      xAxes: [{
        type: 'time',
        time: {
          displayFormats: {
            week: 'YYYY MMM D h:mm:ss'
          }
        }
      }]
    }
  },
  plugins: [{
    beforeDraw: function(chart) {
      var ctx = chart.chart.ctx;
      var ruleIndex = 0;
      var rules = chart.chart.options.backgroundRules;
      var yaxis = chart.chart.scales["y-axis-0"];
      var xaxis = chart.chart.scales["x-axis-0"];
      var partPercentage = 1 / (yaxis.ticksAsNumbers.length - 1);
      for (var i = yaxis.ticksAsNumbers.length - 1; i > 0; i--) {
        if (yaxis.ticksAsNumbers[i] < rules[ruleIndex].yAxisSegement) {
          ctx.fillStyle = rules[ruleIndex].backgroundColor;
          ctx.fillRect(xaxis.left, yaxis.top + ((i - 1) * (yaxis.height * partPercentage)), xaxis.width, yaxis.height * partPercentage);
        } else {
          ruleIndex++;
          i++;
        }
      }
    }
  }]
});
addline(33);
function addline(num){
   var item={
        order: myLineChart.data.datasets.length-1,
        label: 'Title',
        type: 'line',
        borderColor: window.chartColors.green,
        data:new Array(myLineChart.data.datasets[0].data.length).fill(num)
   }
   myLineChart.data.datasets.push(item)
   myLineChart.update();
}
<canvas id="bmi-chart"></canvas>
<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.9.3/Chart.bundle.min.js" integrity="sha256-TQq84xX6vkwR0Qs1qH5ADkP+MvH0W+9E7TdHJsoIQiM=" crossorigin="anonymous"></script>

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