Humans use pressure sense, muscle memory, and rapid visual feedback to make consistent pencil markings on paper. It follows then that my first thought on how to automate this process is to use a force sensor, a three axis position control gantry, and a camera.

It may be possible to indirectly predict force applied between the graphite tip and paper based on a known position of the graphite tip relative to the paper. The challenging part of this approach would be knowing where the tip of graphite is even when every distance traveled on paper wears away some of the tip. The graphite tip wear behavior would have to be studied well enough to be able to make good predictions of how much tip wear occurs per unit length the pencil travels on paper. I imagine graphite tip wear increases with increased force between the pencil and paper as well. 

Fixing the orientation of the pencil relative to the paper and using a force sensitive resistor is going to be the first pencil force measurement method I try. It seems to be a simpler approach than tracking the position of the graphite tip. 

Interestingly, I found a force sensitive resistor that is used in stylus pens for purchase online. It has 5 to 500g sensitivity and shows near linear 1/resistance response to force input.