this happened to me and i discovered that the arial font file had been disabled. first check to see if Arial.ttf has been moved to the disabled fonts directory. from the terminal:
ls /Library/Fonts Disabled
if so, move it back to the active fonts directory.
sudo mv /Library/Fonts Disabled/Arial.ttf /Library/Fonts
log out, log in, then open the ‘Font Book’ application. in my case, arial was present before hand, but it was the Microsoft version stored in /Library/fonts/Microsoft/. Font Book may now show a yellow triangle next to the font name, indicating that multiple copies of the font exist. highlight the font name and chose the Resolve Duplicates command from the Edit menu. this should disable the Microsoft copy, which you can confirm by selecting one of the arial type faces (click the expand triangle next to the font name), right-click on the one labelled Off, and choose Reveal in Finder, which should open a window to Microsoft fonts directory.
So I bumped into the same problem with a code that was working just a few weeks before, and no massive update of anything on the computer (except maybe the OS, now that I get to think about it…).
The way I solved it is that I forced the graphic window to open first by calling
quartz()
before my graphs, and it did the trick. Still unsure about the font, I seem to have the Arial in place.