Plotting a function graph with JavaScript

Plotting a function graph

Question: How do I plot a graph of a mathematical function using JavaScript?

Answer: To plot a graph of a function on your webpage, use the canvas element, as shown in the example below. The canvas element is supported in all major browsers: Firefox, Opera, Safari, Google Chrome, and Microsoft Internet Explorer 9 or newer.

Example: Here is a graph of the functions sin x (green) and cos 3x (blue).

The graph has been created using the following JavaScript code:

function fun1(x) {return Math.sin(x);  }
function fun2(x) {return Math.cos(3*x);}

function draw() {
 var canvas = document.getElementById("canvas");
 if (null==canvas || !canvas.getContext) return;

 var axes={}, ctx=canvas.getContext("2d");
 axes.x0 = .5 + .5*canvas.width;  // x0 pixels from left to x=0
 axes.y0 = .5 + .5*canvas.height; // y0 pixels from top to y=0
 axes.scale = 40;                 // 40 pixels from x=0 to x=1
 axes.doNegativeX = true;

 showAxes(ctx,axes);
 funGraph(ctx,axes,fun1,"rgb(11,153,11)",1); 
 funGraph(ctx,axes,fun2,"rgb(66,44,255)",2);
}

function funGraph (ctx,axes,func,color,thick) {
 var xx, yy, dx=4, x0=axes.x0, y0=axes.y0, scale=axes.scale;
 var iMax = Math.round((ctx.canvas.width-x0)/dx);
 var iMin = axes.doNegativeX ? Math.round(-x0/dx) : 0;
 ctx.beginPath();
 ctx.lineWidth = thick;
 ctx.strokeStyle = color;

 for (var i=iMin;i<=iMax;i++) {
  xx = dx*i; yy = scale*func(xx/scale);
  if (i==iMin) ctx.moveTo(x0+xx,y0-yy);
  else         ctx.lineTo(x0+xx,y0-yy);
 }
 ctx.stroke();
}

function showAxes(ctx,axes) {
 var x0=axes.x0, w=ctx.canvas.width;
 var y0=axes.y0, h=ctx.canvas.height;
 var xmin = axes.doNegativeX ? 0 : x0;
 ctx.beginPath();
 ctx.strokeStyle = "rgb(128,128,128)"; 
 ctx.moveTo(xmin,y0); ctx.lineTo(w,y0);  // X axis
 ctx.moveTo(x0,0);    ctx.lineTo(x0,h);  // Y axis
 ctx.stroke();
}

It is safe to call the draw() function after the entire body of the webpage is loaded. The source code of an HTML page containing the canvas element may look like this:

<!DOCTYPE html>
<html>
<head><title>Canvas code example</title>
<script type="text/javascript">
// JavaScript source code goes here
</script>
</head>
<body onload="draw()">
<canvas id="canvas" width="502" height="108"></canvas>
</body>
</html>

Note: If you omit the DOCTYPE element at the beginning of the page, then the graphics might not show up in IE browsers: without the DOCTYPE element, IE may switch to the backward compatibility mode and treat canvas as an unsupported tag.