JAVA SCRIPT - Creating a Dynamic Line Chart in Canvas - Supercoders | Web Development and Design | Tutorial for Java, PHP, HTML, Javascript JAVA SCRIPT - Creating a Dynamic Line Chart in Canvas - Supercoders | Web Development and Design | Tutorial for Java, PHP, HTML, Javascript


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Thursday, January 3, 2019

JAVA SCRIPT - Creating a Dynamic Line Chart in Canvas

Creating a Dynamic Line Chart in Canvas


You want to display a line chart in your web page, but the data changes over time, and you want to dynamically update it. 


Use the canvas element and the path method to create the chart. When the data changes, update the chart: 

var array1 = [[100,100], [150, 50], [200,185],
 [250, 185], [300,250], [350,100], [400,250],
 [450, 100], [500,20], [550,80], [600, 120]];
var imgcanvas = document.getElementById("imgcanvas");
if (imgcanvas.getContext) {
 var ctx = imgcanvas.getContext('2d');
 // rect one
 // line path
 for (var i = 0; i < array1.length; i++) {
 ctx.lineTo(array1[i][0], array1[i][1]);


Canvas paths are the way to create arbitrary shapes in Canvas. After getting the canvas context, ctx, the path is begun with a call to ctx.beginPath(), which begins a new Canvas path. The next line of code is ctx.moveTo, which moves the drawing “pen” to a beginning location, but without drawing. From that point, several lineTo() calls are made using an array of paired values representing the x,y location for each line endpoint. 

After the path has been defined, it’s drawn. We’re not creating a closed path, so I’m not using ctx.closePath(), which would draw all the defined lines and then attempt to draw a line from the ending point to the beginning point. Instead, I’m drawing the line given the points that have been defined, using ctx.stroke(). 

The appearance of the drawing is influenced by two Canvas settings: strokeStyle and fillStyle. The strokeStyle setting sets the color for the outline of a drawing, while the fillStyle does the same for the drawing filling:


Any CSS setting will do, or you can use a CanvasGradient or CanvasPattern. You can use the rgba setting to add transparency:


You can also use the globalAlpha setting to set the transparency for any drawing that follows:

ctx.globalAlpha = 0.2;

You can further control the appearance of the drawing outline by changing the stroke’s line width:


To dynamically update the chart, you can incorporate timers, and either replace the path (by creating an entirely new context, which would erase the old), or add the new line chart to the same chart.

Shows a web page that creates the line in the solution and then creates two others, each drawn after a short period of time using timers. The colors for the stroke path are changed between lines.

Using timers to dynamically update a line chart

<!DOCTYPE html>
<title>Canvas Chart</title>
<meta charset="utf-8" />
<canvas id="imgcanvas" width="650" height="350">
<p>Include an image that has a static representation of the chart</p>
 var points = [[[100,100], [150, 50], [200,185],
 [250, 185], [300,250], [350,100], [400,250],
 [450, 100], [500,20], [550,80], [600, 120]],
 [[100,100], [150, 150], [200,135],
 [250, 285], [300,150], [350,150], [400,280],
 [450, 100], [500,120], [550,80], [600, 190]],
 [[100,200], [150, 100], [200,35],
 [250, 185], [300,10], [350,15], [400,80],
 [450, 100], [500,120], [550,80], [600, 120]]];
 var colors = ['black','red','green'];
 var imgcanvas = document.getElementById("imgcanvas");
 if (imgcanvas.getContext) {
 var ctx = imgcanvas.getContext('2d');
// rectangle wrapping line chart
 points.forEach(function(element, indx, arry) {
 setTimeout(function() {
 // set up beginning
 ctx.strokeStyle = colors[indx];
 for (var i = 0; i < element.length; i++) {
 ctx.lineTo(element[i][0], element[i][1]);
 }, indx * 5000);

Simplify Your Canvas Charts Using a Library

It doesn’t have to be difficult to create a chart using Canvas from scratch, but why walk the steps taken by others? There are several excellent libraries that can simplify not only chart making but other Canvas effects. Over 50 libraries for chart making are listed in a TechSlides Page, including the in‐ creasingly popular D3, which I’ll cover in . 

Most of the libraries are freely available, though some do charge a fee for commercial use. One of the libraries, Highcharts, even provides demonstrations that you can edit in jsFiddle, making it easy to try out the library’s capability. It’s dependent on jQuery, reducing the code to an absurdly simple level. As an example, one of the demonstrations is for a very professional line chart, with plot lines and labels, 

Yet the code to create this example is equivalent to that in the following code block, which I modified to feature my own locations and temperature metric, which you can try yourself at jsFiddle:

$(function () {
 title: {
 text: 'Monthly Average Temperature',
 x: -20 //center
 subtitle: {
 text: 'Source:',
 x: -20
 xAxis: {
 categories: ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
 yAxis: {
 title: {
 text: 'Temperature (°F)'
 plotLines: [{
 value: 0,
 width: 1,
 color: '#808080'
 tooltip: {
 valueSuffix: '°F'
 legend: {
 layout: 'vertical',
 align: 'right',
 verticalAlign: 'middle',
 borderWidth: 0
 series: [{
 name: 'Seattle, WA',
 data: [47,51,55,59,65,70,75,75,70,60,52,47]
 }, {
 name: 'Grand Isle, VT',
 data: [27,31,40,54,67,76,81,79,71,57,45,33]
 }, {
 name: 'St. Louis, MO',
 data: [40,45,55,67,77,85,89,88,81,69,56,43]

Not only is the plotted chart professional looking, it’s zoomable, which means you can move your mouse cursor over the chart to examine the plot points in detail. That level of interactivity isn’t necessarily trivial in Canvas, because one of the downsides to Canvas is the fact that you can’t attach event handlers to the individual elements of Canvas— only to the Canvas area itself.

Not being able to attach an event to individual elements means that you’ll have to keep track of where the mouse is, and what’s underneath it at any point in time.

Thankfully, you can attach event handlers to SVG elements, as demonstrated.

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