From the tutorial at Adobe's site,

scale(<sx> [<sy>]) will scale the element by factor of <sx> and <sy> f
+rom 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 ele
+ment will be scaled to 50% of the original width. In the full six-num
+ber array, <sx> and <sy> are the first and fourth numbers of the arra
+y. 

To scale using a centerpoint other than (0,0), you must translate the 
+element so that the centerpoint becomes (0,0), perform the scale, the
+n translate the element back to its original location. For example, t
+o scale an element by 50% centered on (150,100), you must set the tra
+nsform attribute to "translate(150 100) scale(.5) translate(- 150 -10
+0)" (note the reverse order).
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