I want to move A, B, C, D labels so that they are akin sub "x-strips". Basically move them leftward, to the top of the plots hanging above the corresponding X and Y plots. Any suggestions on how to do this?
set.seed(14)
df <- data.frame(
  var.test = rep(c("X","Y"),each =  40),
  var.test2 = rep(c("A","B","C","D"), times = 20),
  val.test = rnorm(80,4,5),
  x = c(1:80)
)
my.formula <- c(y~x,y~x,y~x,y~x,y~x + I(x^2), y~x + I(x^2),y~x + I(x^2), y ~x)
ggplot(df, aes(x = x, y = val.test)) + 
  geom_point() + 
  facet_grid(var.test2~var.test)+ theme(strip.text.y = element_text(angle = 360))

