$ctx is a jQuery object. use $ctx[0].getContext('2d') to get the context
$ctx
$ctx[0].getContext('2d')
If using excanvas you will need to use the following so it works in IE.
var canvas = $ctx[0];if (canvas.getContext == undefined) { return G_vmlCanvasManager.initElement(canvas).getContext("2d"); }return canvas.getContext('2d')
first thing first , you need to check out if the following line is exist and the top of HTML document.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML Strict//EN">
this line as a browser render policy excuted condition.