We can add any of these to a plot and override existing

For datasets, use %+%

dataset1 <- data.frame(id = rep(letters[1:3], each = 100),
                       Value = c(rnorm(100, mean = 1),
                                 rnorm(100, mean = 2),
                                 rnorm(100, mean = 3)))

dataset2 <- data.frame(id = rep(letters[1:3], each = 100),
                       Value = c(rpois(100, lambda = 1),
                                 rpois(100, lambda = 2),
                                 rpois(100, lambda = 3)))
p1 <- ggplot(dataset1, aes(x = Value, col = id))+
  geom_density()+
  theme_minimal()
p1

p2 <- p1 %+% dataset2
p2