JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.1.4/Chart.min.js"></script>
<canvas id="myChart" width="400" height="400"></canvas>

JavaScript

var ctx = document.getElementById("myChart");

var lineSuperDraw = Chart.controllers.line.prototype.draw;
Chart.helpers.extend(Chart.controllers.line.prototype, {
  draw : function() {
    var chart = this.chart;
    var ctx = chart.chart.ctx;

    var yRangeBegin = chart.config.data.yRangeBegin;
    var yRangeEnd = chart.config.data.yRangeEnd;

    var xaxis = chart.scales['x-axis-0'];
    var yaxis = chart.scales['y-axis-0'];

    var yRangeBeginPixel = yaxis.getPixelForValue(yRangeBegin);
    var yRangeEndPixel = yaxis.getPixelForValue(yRangeEnd);

    ctx.save();

    for (var yPixel = Math.min(yRangeBeginPixel, yRangeEndPixel); yPixel <= Math.max(yRangeBeginPixel, yRangeEndPixel); ++yPixel) {
      ctx.beginPath();
      ctx.moveTo(xaxis.left, yPixel);
      ctx.strokeStyle = '#00ff00';
      ctx.lineTo(xaxis.right, yPixel);
      ctx.stroke();
    }

    ctx.restore();

    lineSuperDraw.apply(this, arguments);
  }
});

var myChart = new Chart(ctx, {
  type: 'line',
  data: {
    labels: ["Red", "Blue", "Yellow", "Green", "Purple", "Orange"],
    datasets: [{
      label: '# of Votes',
      data: [12, 19, 3, 5, 2, 3],
    }],
    yRangeBegin : 6.5,
    yRangeEnd : 12.5
  },
  options: {
    scales: {
      yAxes: [{
        ticks: {
          beginAtZero:true
        }
      }]
    }
  }
});