I would render the pencil alone and then I would animate a mask over the 2D lines in a composing program.
You could otherwise use align to spline to animate the pencil. Then use that same spline with in a sweep nurbs that has the growth at the same rate as your pencil.
Another option is to use a Mograph tracer object on the tip of the pencil.
I guess there are a few more options for this too.
/Randy