sorry, I was wrong. I think the thing to do, is not bother with naming NA factor level, just name your other levels, when you go to do the table though, use the helper function addNA
charvec <- c(rep("a",3),
rep("b",4),
rep(NA,5))
factorvec <- factor(charvec, levels=c("a","b") )
levels(factorvec)
fvt <- table(addNA(factorvec , ifany=TRUE))
fvt
barplot(fvt, col="blue",
xlab="Degree",
ylab="Number",
border="red",
names.arg =c ("a","b","NA"))![Rplotexample|478x477]
