Willem, Edzer, and Gstat list,
I'm teaching a class that uses R in the lab and was hammered by the
plot.variogram() problem two weeks ago; it was especially nasty since
plotting worked fine on my machine but not in the lab, and I had been too
lazy to test my homework code in the lab.
While plot.variogram failed, I was able to work around the problem by simply
using plot(). R to the rescue - it appears that plot() is smart enough to
recognize the gstat variogram object and plots it properly - it worked on
omnidirectional, directional, and variogram model objects. An advantage is
that you do not need to de- and re- attach geoR or other libraries, and I
assume you could also print their objects.