Source: http://jsfiddle.net/Lgtzcr7v/2/

Horizontal annotation line.js

var horizonalLinePlugin = {
  afterDraw: function(chartInstance) {
    var yScale = chartInstance.scales["y-axis-0"];
    var canvas = chartInstance.chart;
    var ctx = canvas.ctx;
    var index;
    var line;
    var style;

    if (chartInstance.options.horizontalLine) {
      for (index = 0; index < chartInstance.options.horizontalLine.length; index++) {
        line = chartInstance.options.horizontalLine[index];

        if (!line.style) {
          style = "rgba(169,169,169, .6)";
        } else {
          style = line.style;
        }

        if (line.y) {
          yValue = yScale.getPixelForValue(line.y);
        } else {
          yValue = 0;
        }

        ctx.lineWidth = 3;

        if (yValue) {
          ctx.beginPath();
          ctx.moveTo(0, yValue);
          ctx.lineTo(canvas.width, yValue);
          ctx.strokeStyle = style;
          ctx.stroke();
        }

        if (line.text) {
          ctx.fillStyle = style;
          ctx.fillText(line.text, 0, yValue + ctx.lineWidth);
        }
      }
      return;
    };
  }
};

Chart.pluginService.register(horizonalLinePlugin);

Usage.js

var myChart = new Chart(ctx, {
  type: 'line',
  data: data,
  options: {
    "horizontalLine": [{
      "y": 82,
      "style": "rgba(255, 0, 0, .4)",
      "text": "max"
    }, {
      "y": 60,
      "style": "#00ffff",
    }, {
      "y": 44,
      "text": "min"
    }]
  }
});