here is another approach by subsetting data:
ggplot(mtcars, aes(mpg, disp)) + facet_wrap(~cyl) +
geom_point(data = subset(mtcars, cyl == 4)) +
geom_line(data = subset(mtcars, cyl == 6)) +
geom_text(data = subset(mtcars, cyl == 8), aes(label = gear))
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…