Line break in ggplot annotate with LateX expression

You could use atop and plotmath expressions instead (see ?plotmath for additional info):

ggplot(NULL, aes(c(-5,5))) +  geom_area(stat = "function", fun = dnorm, fill = "grey70", xlim = c(-2, 2)) +  annotate(geom = "text", label = expression(atop("Distribution of"~bar(x), "under"~H[0])),           x = -1, y = 0.3,           color = "red") +  theme_classic()

I've changed the theme and colors for this example so that the text will stand out.

UPDATE: Regarding the comment, here's one option, though you'll need to play around with the vertical spacing. We first construct exp, a list of plotmath expressions. Then, in annotate, we need y to be a vector of values equal in length to the number of elements in exp. parse=TRUE tells ggplot to treat the elements of exp as plotmath expressions and parse them:

exp = list("Distribution of"~bar(x),           "under"~H[0],           hat(mu)~"is the mean")ggplot(NULL, aes(c(-5,5))) +  geom_area(stat = "function", fun = dnorm, fill = "grey70", xlim = c(-2, 2)) +  annotate(geom = "text", label = exp,           x = -1, y = seq(0.32,0.28,length=3),           size=3, color = "red", parse=TRUE) +  theme_classic()