They expect diagrams drawn in pencil
Lot's of people do the entire exam in pencil - despite the guidance being to use pen.
It will appear on the scan.
A hard pencil, tiny writing and hardly pressing down as you write and it might be hard to see clearly.
A bigger problem is that pencil does not erase as well as you think and scanners are so sensitive that it can find the erased stuff as well as the new stuff sometimes making heavily revised work hard to read.