Add a geom layer for a single panel in a faceted plot

Question

Taking a cue from the following link Aligning two plots with ggplot2, I was able to plot 2 "y" variables faceted against a common x axis. What I want to do now is to be able to add a geom_point layer to only one of the facets. This layer uses a different dataset(d3) with a same structure as d1. When I add the layer it gets used on both facets. Is it possible to layer the points only the upper facet.

library(ggplot2)

x <- seq(1992, 2002, by = 2)
d1 <- data.frame(x = x, y = rnorm(length(x)))
xy <- expand.grid(x = x, y = x)
d2 <- data.frame(x = xy$x, y = xy$y, z = jitter(xy$x + xy$y))
d3 <- data.frame(x = x, y = 3+rnorm(length(x)))

d1$panel <- "a"
d2$panel <- "b"
d1$z <- d1$x

d <- rbind(d1, d2)

p <- ggplot(data = d, mapping = aes(x = x, y = y))
p <- p + facet_grid(panel ~ ., scale = "free")
p <- p + layer(data = d1,  geom = c( "line"), stat = "identity")
###*p <- p + layer(data = d3,  geom = c( "point"))* - This is the layer I intend to add only to the top panel

p <- p + layer(data = d2,  geom = "line", stat = "identity")
p

Answer 1

Just add the panel column to d3 with the panel you want to add the point set to. In your case:

d3$panel = "a"

p <- ggplot(data = d, mapping = aes(x = x, y = y))
p <- p + facet_grid(panel ~ ., scale = "free")
p <- p + layer(data = d1,  geom = c( "line"), stat = "identity")
p <- p + layer(data = d3,  geom = c( "point"))
p <- p + layer(data = d2,  geom = "line", stat = "identity")
p

which yields the correct output:

If the column mentioned in the call to facet_grid is not present, ggplot2 assumes it needs to be printed on all facets. When you specify panel, ggplot2 will take it into account.