Simplest way to plot changes in ranking between two ordered lists in R?

Question

I'm wondering if there is an easy way to plot the changes in position of elements between 2 lists in the form of a directed bipartite graph in R. For example, list 1 and 2 are vectors of character strings, not necessarily containing the same elements:

list.1 <- c("a","b","c","d","e","f","g")

list.2 <- c("b","x","e","c","z","d","a")

I would like to generate something similar to:

I've had a slight bash at using the igraph package, but couldn't easily construct what I would like, which I imagine and hope shouldn't be too hard.

Cheers.

Answer 1

Here is a simple function to do what you want. Essentially it uses match to match elements from one vector to another and arrows to draw arrows.

plotRanks <- function(a, b, labels.offset=0.1, arrow.len=0.1)
  {
  old.par <- par(mar=c(1,1,1,1))

  # Find the length of the vectors
  len.1 <- length(a)
  len.2 <- length(b)

  # Plot two columns of equidistant points
  plot(rep(1, len.1), 1:len.1, pch=20, cex=0.8, 
       xlim=c(0, 3), ylim=c(0, max(len.1, len.2)),
       axes=F, xlab="", ylab="") # Remove axes and labels
  points(rep(2, len.2), 1:len.2, pch=20, cex=0.8)

  # Put labels next to each observation
  text(rep(1-labels.offset, len.1), 1:len.1, a)
  text(rep(2+labels.offset, len.2), 1:len.2, b)

  # Now we need to map where the elements of a are in b
  # We use the match function for this job
  a.to.b <- match(a, b)

  # Now we can draw arrows from the first column to the second
  arrows(rep(1.02, len.1), 1:len.1, rep(1.98, len.2), a.to.b, 
         length=arrow.len, angle=20)
  par(old.par)
  }

A few example plots

par(mfrow=c(2,2))
plotRanks(c("a","b","c","d","e","f","g"),
          c("b","x","e","c","z","d","a"))
plotRanks(sample(LETTERS, 20), sample(LETTERS, 5))
plotRanks(c("a","b","c","d","e","f","g"), 1:10) # No matches
plotRanks(c("a", "b", "c", 1:5), c("a", "b", "c", 1:5)) # All matches
par(mfrow=c(1,1))