angular-chart add horizontal line

First of all, you need to extends Chart.js with the extend you mentionned in your post, like this : (using @jbman223 snippet)

// Extend chart.js with a new type of chart
Chart.types.Line.extend({
   name: "LineWithLine",
   initialize: function () {
      Chart.types.Line.prototype.initialize.apply(this, arguments);
   },
   draw: function () {
    Chart.types.Line.prototype.draw.apply(this, arguments);

    // Needs to be set with angular-chart options
    var lineAtIndex = 2;

    var point = this.datasets[0].points[lineAtIndex]
    var scale = this.scale
    console.log(this);

    // draw line
    this.chart.ctx.beginPath();
    this.chart.ctx.moveTo(scale.startPoint+12, point.y);
    this.chart.ctx.strokeStyle = '#ff0000';
    this.chart.ctx.lineTo(this.chart.width, point.y);
    this.chart.ctx.stroke();

    // write TODAY
    this.chart.ctx.textAlign = 'center';
    this.chart.ctx.fillText("TODAY", scale.startPoint + 35, point.y+10);
}
});

Then, we have to link this new Chart type with angular-chart. Unfortunately, it is a high level of abstraction library, so there is no built-in feature for that. So, the only way I have found to do that so far is to modify angular-chart.js by adding a line in the config :

  return angular.module('chart.js', [])
  .provider('ChartJs', ChartJsProvider)
  .factory('ChartJsFactory', ['ChartJs', '$timeout', ChartJsFactory])
  //...
  .directive('chartLinebis', ['ChartJsFactory', function (ChartJsFactory) { return new ChartJsFactory('LineWithLine'); }]);

Finally, call angular-chart with your new chart label :

      <canvas class="chart chart-linebis" chart-data="data" chart-labels="labels" chart-legend="true" chart-series="series"></canvas>

Note that it is very important that the js imports respect this order : chart.js -> myExtend.js -> angular-chart.js

JSFiddle (disclaimer : I included angular-chart.js in the middle of the snippet for import order purpose)