The unicode combining overline character, U+0305 is useful for indicating mathematical terms such as X̅ for the mean of a random variable X. (Some prefer the appearance of the macron, U+0304 (X̄), but a quick Wikipedia suggests this is imprecise.)
I understand how to enter such a character in R, and
cat("X\u0305")
correctly produces X̅. However, when I attempt to place this character in a plot(1:10,10:1,main="X\u0305")
the overline is dramatically offset, as shown here. The same issue is present in ggplot, as
ggplot(data.frame(x=1:10,y=10:1)) + geom_point(aes(x=x,y=y,color="X\u0305"))
produces similar output.
I am running Mac OS X El Capitan, and I encounter this problem in both the Terminal and in RStudio. The similarly tagged question, "Unicode characters in ggplot legend" did not solve my problem.
Another option would be to use expression(bar(X)):
ggplot(data.frame(x=1:10,y=10:1)) +
geom_point(aes(x=x,y=y,color="Xbar")) +
scale_colour_discrete(labels=expression(bar(X)))
plot(1:10,10:1,main=expression(bar(X)))
?plotmath has more info on the various options for mathematical text using expressions in R.