Questions on Slider.ValueChanged

From before, I'm trying to write a sinewave graphing program. I want
the wave to move across the screen with time, and I want the speed at
which it moves to be controlled by a slider.

Right now, I have an EventTrigger-Storyboard combination inside my
slider's triggers property; the event trigger fires on ValueChanged, and
it starts a storyboard that has a DoubleAnimation that increases the
sinewave's Time property; my problem is that when ValueChanged fires,
the value hasn't changed yet; if I drag it, the problem isn't noticable,
but if I click on the rail (to make the slider jump), the wavespeed
stays the same. I click again, the thing goes to the speed it *should*
have been at before.

How can I accomplish what I'm looking to do? Am I on the right track?
Should I be doing something else?

Thanks,
Nick