scale(<sx> [<sy>]) will scale the element by factor of <sx> and <sy> from the centerpoint (0,0). If <sy> is not given, it is assumed to be the same as <sx>. For example, if the value of <sx> is "0.5," the element will be scaled to 50% of the original width. In the full six-number array, <sx> and <sy> are the first and fourth numbers of the array. 

To scale using a centerpoint other than (0,0), you must translate the element so that the centerpoint becomes (0,0), perform the scale, then translate the element back to its original location. For example, to scale an element by 50% centered on (150,100), you must set the transform attribute to "translate(150 100) scale(.5) translate(- 150 -100)" (note the reverse order).