paired data for a facet_wrap - r

Question

Imagine I have data foo below. Each row contains a measurement (y) on a species and each species is paired with another (species.pair). So in the example below, species a is paired with e, b with f, and so on. The number of observations for each species varies. I'd like to plot the density of each species's distribution along with its partner's distribution in its own facet. Below I hand coded this with the column sppPairs. The species are all unique and each has a match in species.pair. I'm unsure of how to make the grouping column sppPairs below. I'm sure there is some clever way to do this with {dplyr} but I can't figure out what to do. Some kind of pasting species to species.pair I imagine? Any help much appreciated.
foo <- data.frame(species = rep(letters[1:8],each=10),
species.pair = rep(letters[c(5:8,1:4)],each=10),
y=rnorm(80))
# species and species pair match exactly
all(unique(foo$species) %in% unique(foo$species.pair))
# what I want
foo$sppPairs <- c(rep("a:e",10),
rep("b:f",10),
rep("c:g",10),
rep("d:h",10),
rep("a:e",10),
rep("b:f",10),
rep("c:g",10),
rep("d:h",10))
p1 <- ggplot(foo,aes(y,fill=species))
p1 <- p1 + geom_density(alpha=0.5)
p1 <- p1 + facet_wrap(~sppPairs)
p1

Answer 1

Yes, you can use apply on the appropriate columns to paste the sorted elements together in the correct order (otherwise a:e is different from e:a and so on, and you end up with 8 groups instead of 4):
library(ggplot2)
foo <- data.frame(species = rep(letters[1:8], each = 10),
species.pair = rep(letters[c(5:8, 1:4)], each = 10),
y = rnorm(80))
foo$sppPairs <- apply(foo[c("species", "species.pair")], 1,
function(x) paste(sort(x), collapse = ":"))
ggplot(foo, aes(y, fill = species)) +
geom_density(alpha = 0.5) +
facet_wrap(~sppPairs)
Created on 2020-10-05 by the reprex package (v0.3.0)