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 t

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.