taper Ob to affect only 1 axis during animation?

I’m using the taper ob for a curve and want to animate the length of the extruded bevel.
I have used a tutorial on vimeo 16744767 to rig the bevel with a hook (i think those are the correct terms) to enable me to animate the length but the width of my original bevelled object seems to more than 100% at the start of the curve when i get to the extent of the “graph”.

is there a way to use the taper ob to only alter the length of the curve?
Or have i done something wrong anyway?

Cheers for any help.