JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://rawgit.com/nnnick/Chart.js/v1.0.2/Chart.min.js"></script>
<canvas id="myChart" height="300" width="800"></canvas>

JavaScript

var data = {
        labels: ["JAN", "FEB", "MAR", "APR", "MAY", "JUN", "JUL", "AUG", "SEP", "OCT", "NOV", "DEC"],
        datasets: [{
            data: [12, 3, 2, 1, 8, 8, 2, 2, 3, 5, 7, 1]
        }]
    };

    var ctx = document.getElementById("myChart").getContext("2d");
    Chart.types.Line.extend({
        name: "LineWithYLine",
        draw: function () {
            Chart.types.Line.prototype.draw.apply(this, arguments);

            var scale = this.scale
            var ctx = this.chart.ctx;
            // calculate using the scale
            var y = scale.calculateY(this.options.lineAtValue);

            // draw line
            ctx.save();
            ctx.strokeStyle = '#ff0000';
            ctx.beginPath();
            ctx.moveTo(Math.round(scale.xScalePaddingLeft), y);
            ctx.lineTo(this.chart.width, y);
            ctx.stroke();
            ctx.restore();
        }
    });

    new Chart(ctx).LineWithYLine(data, {
        datasetFill: false,
        lineAtValue: 7.5
    });