密度图矩阵，每个图覆盖两个分布

Question

I have a data.frame with 5 columns and I'd like to generate a matrix of density plots, such that each density plot is an overlay of two density plots. (This is akin to plotmatrix, except that in my case, the number of non-NA value in each column differ from column to column and I want overlaid distributions rather than scatter plots).

My first attempt, which didn't work, is given below:

library(ggplot2)
library(reshape)

tmp1 <- data.frame(do.call(cbind, lapply(1:5, function(x) {
  r <- rnorm(100)
  r[sample(1:100, 20)] <- NA
  return(r)
})))

ggplot( melt(tmp1), aes(x=value, fill=variable))+
  geom_density(alpha=0.2, position="identity")+opts(legend.position = "none")+
  facet_grid(variable ~ variable)

My second approach got me nearly there, but instead of 5 different colors, I only want to use two colors across all the plots. And, I'm sure there is a more elegant way to construct this expanded matrix:

tmp2 <- do.call(rbind, lapply(1:5, function(i) {
  do.call(rbind, lapply(1:5, function(j) {
    r <- rbind(data.frame(var=sprintf('X%d', i), val=tmp1[,i]),
               data.frame(var=sprintf('X%d', j), val=tmp1[,j]))
    r <- data.frame(xx=sprintf('X%d', i), yy=sprintf('X%d', j), r)
    return(r)
  }))
}))
ggplot(tmp2, aes(x=val, fill=var))+
  geom_density(alpha=0.2, position="identity")+opts(legend.position = "none")+
  facet_grid(xx ~ yy)

My solution was to manually loop through the pairs of columns and generate the overlaid density plots by hand, saving them to a list. I then arranged them in a grid using `grid.arrange' giving the image below.

But is it possible to achieve this using facet_grid instead?

Answer 1

The easiest way is to reshape your data with all permutations (5 * 5 = 25 of them).

require(gregmisc)
perm <- permutations(5, 2, paste0("X", 1:5), repeats.allowed=TRUE)
# instead of gregmisc + permutations, you can use expand.grid from base as:
# perm <- expand.grid(paste0("X", 1:5), paste0("X", 1:5))
o <- apply(perm, 1, function(idx) {
    t <- tmp1[idx]
    names(t) <- c("A", "B")
    t$id1 <- idx[1]
    t$id2 <- idx[2]
    t
})
require(ggplot2)
require(reshape2)    
o <- do.call(rbind, o)
o.m <- melt(o, c("id1", "id2"))
o.m$id1 <- factor(o.m$id1)
o.m$id2 <- factor(o.m$id2)
p <- ggplot(o.m, aes(x = value))
p <- p + geom_density(alpha = 0.2, position = "identity", aes(fill = variable)) 
p <- p + theme(legend.position = "none")
p <- p + facet_grid(id1 ~ id2)
p