If you want to find angle between vectors (point-previous) and (point-now), use:
Math.atan2((now.x - point.x)*(previous.y - point.y) -
(now.y - point.y)*(previous.x - point.x),
(now.x - point.x)*(now.y - point.y) -
(previous.x - point.x)*(previous.y - point.y))
(This is atan2(vectorproduct, scalarproduct)
)
You may also use
angle = atan2(now.y-point.y, now.y-point.y)-
atan2(previous.y-point.y, previous.x-point.x);