Make arbitrary line dashed

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Just wrap the block where you set the ctx properties and draw with the following lines to save and restore the context,

this.chart.ctx.save();
...
this.chart.ctx.restore();

Fiddle – http://jsfiddle.net/ps3186ex/

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("LineWithLine").getContext("2d");

Chart.types.Line.extend({
  name: "LineWithLine",
  draw: function() {
    Chart.types.Line.prototype.draw.apply(this, arguments);

    var point = this.datasets[0].points[this.options.lineAtIndex]
    var scale = this.scale

    // draw line
    this.chart.ctx.save();
    this.chart.ctx.setLineDash([3]);
    this.chart.ctx.beginPath();
    this.chart.ctx.moveTo(point.x, scale.startPoint + 24);
    this.chart.ctx.strokeStyle = '#ff0000';
    this.chart.ctx.lineTo(point.x, scale.endPoint);
    this.chart.ctx.stroke();
    this.chart.ctx.restore();

    // write TODAY
    this.chart.ctx.textAlign = 'center';
    this.chart.ctx.fillText("TODAY", point.x, scale.startPoint + 12);
  }
});

new Chart(ctx).LineWithLine(data, {
  datasetFill: false,
  lineAtIndex: 2
});
<script src='https://rawgit.com/nnnick/Chart.js/v1.0.2/Chart.min.js'></script>
<canvas id="LineWithLine" width="600" height="400"></canvas>

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