I'm having problems with quadratic bezier curves

I'm using the Canvas plugin and I'm trying to split an quadratic bezier curve, but the result is a little off.

dl.dropboxusercontent.com/u/75048835/splitbezier/index.html

(use left and right to change the ratio)

Place it on the middle and you'll see what I'm talking about.

Basically this is what I'm trying to do:

<img src="http://philippe.elsass.me/wp-content/uploads/2009/06/bezier.png" border="0">

Capx here