code: function(type, pos, width, height, canvas){
        canvas.getCtx()[type + "Rect"](pos.x - width / 2, pos.y - height / 2, 
                                        width, height);
      }
docs: /*
  Method: render
  
  Renders a rectangle into the canvas.
  
  Parameters:
  
  type - (string) Possible options are 'fill' or 'stroke'.
  pos - (object) An *x*, *y* object with the position of the center of the rectangle.
  width - (number) The width of the rectangle.
  height - (number) The height of the rectangle.
  canvas - (object) A <Canvas> instance.
  
  Example:
  (start code js)
  NodeHelper.rectangle.render('fill', { x: 10, y: 30 }, 30, 40, viz.canvas);
  (end code)