Graffiti sur supports divers

Pictures of graffiti on Flickr:
Gladiator Graffiti

With Graffiti we have to be very carefull as these were mostly scratched into a stone surface.
Sometimes scratches and damages of later date might actually be included in some line drawings.
I am currently looking for a picture of the "Essedarius" line drawing in the Junkelmann book.
In the drawing it shows a straight line running behind the oval Scutum and Junkelmann supposes this might be a javelin used by the Essedarii/Galli at the beginning of the combat.
Recently I saw the origial very shortly in a BBC documentation at I had the impression that this line was actually a damage to the surface and not included in the original grafitti.