This can be done using the effect > generate > write on. You want to change the "Brush Time Properties" to "Size". Then keyframe the brush size as you move animate the drawing of the line.
Quick example, using my bad hand-writing:
7217_writeonexample.aep.zip
Johnny Cuevas, Editor
Thinkck.com
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.