Sherlock draws, or rather drafts. It shouldn't be a surprise because he's studied biology and it's often part of the studies to draw cross sections and studies of muscles, bones, tumors. John struggled with that part of it, never quite getting the hang.
But Sherlock draws dark, clean lines on heavy paper, coarse paper, describing body parts and spores and other organic things.
And it occurs to John that Sherlock makes clean lines in his mind between the scuff mark two-thirds of the way down the door and the reason for it being there. And in that, Sherlock is an artist.