But graffiti...that's a good point, never would have thought of it myself. But now you've obliged me to do so...let's see:
We know Roman soldiers had to speak Latin, we know Roman soldiers were mostly posted far away fom their homes, we know soldiers far from their homes are inveterate graffiti-ists.
We know the whole population of Britain, France, Italy, Spain and Portugal either spoke an unwritten language and/or Latin. Therefore all graffiti-ists in Britain, France, Italy, Spain and Portugal would be obliged to use Latin.
I begin to see why I didn't think of it.