[Быдло]

А если так?

avg_y = 0;


for (i=0; i<_root.yArray.length; i++) {

avg_y = Number(avg_y) + Number(_root.yArray[i]);
trace (avg_y);
trace (Number(avg_y));
if (_root.yArray[i] > max_y) {max_y=_root.yArray[i]};
if (_root.yArray[i] < min_y) {min_y=_root.yArray[i]};

}