Chart.js — drawing an arbitrary vertical line

Question

How can I draw an vertical line at a particular point on the x-axis using Chart.js?

In particular, I want to draw a line to indicate the current day on a LineChart. Here's a mockup of the chart: http://i.stack.imgur.com/VQDWR.png

Answer 1

Update - this answer is for Chart.js 1.x, if you are looking for a 2.x answer check the comments and other answers.

You extend the line chart and include logic for drawing the line in the draw function.

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Preview

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HTML

<div>     <canvas id="LineWithLine" width="600" height="400"></canvas> </div> 

Script

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.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();          // 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 }); 

The option property lineAtIndex controls which point to draw the line at.

Fiddle - http://jsfiddle.net/dbyze2ga/14/