dat = data.frame(x = 1:4, y = ordered(c(4,'>5',1,1), levels = c(1:5, '>5')), grp = 1)
ggplot(dat, aes(x, y, group=grp)) + geom_step() + 
  scale_y_discrete(breaks = levels(dat$y), labels = levels(dat$y))
ggplot(dat, aes(x, y, group=grp)) + geom_step() + 
  scale_y_discrete(breaks = levels(dat$y), labels = levels(dat$y), drop = FALSE)