Function to break up plotted tree into multiple columns of figures

In early October, a R-sig-phylo query was submitted that read as follows:

I'm wondering if there is a way to plot a phylogeny in R that is broken up and displayed side by side to condense space. I've made a backbone in R that i'd like to make into a figure, but it is too tall. If this isn't the write setup, could someone suggest what program (and format for me to write.tree) from R. I am using windows, in terms of potential third part programs.

Well, I thought this sounded like a neat idea. We often see trees plotted this way in published articles, but there is no native function in R to produce this kind of plot. Unfortunately, I didn't have time to work on this until recently - so most likely, the author of the R-sig-phylo question has already solved this problem in some other way. Nonetheless. . . . .

(I should note that, in discussing this function with my friend Luke Harmon, he warned me that I was stuck in the "paper paradigm" - for more on what that means, see Luke's article with James Rosindell.)

Ignoring Luke (as is my wont), I just posted a function for this anyway. The function, splitplotTree, can be downloaded from my phytools page (direct link to code here).

A little bit about how I did this. . . . To plot a phylogram we need two coordinate matrices: a matrix for the heights of the nodes above the root; and a matrix for the vertical position of the edges. We get the former for a "phylo" object by using (say) nodeHeights in phytools. To get the latter, we can just evenly space the tip nodes on our vertical axis (for a rightward facing tree); and then work down through their common ancestors to the root, averaging the vertical position of the descendant nodes to obtain the height of each parent.

Having done this, to split the tree horizontally we just have to select a cutting point. Then we split our matrices based on this point. After some adjustments, we plot our trees in each plot window. Finally, we have to plot the vertical lines (the lines connecting daughter edges) that would normally connect the two plots. This is easy. For the upper panel tree slice, we just plot a line from each unmatched node to the lower edge of the plot; and for the lower panel tree, we do the opposite. The result will look something like this:

> library(phytools)
> source("splitplotTree.R")
> tree<-pbtree(n=100)
> splitplotTree(tree,fsize=0.7,ftype="i",split=0.5)

We might look at this and decide that we don't like the look of the slice point split and would prefer to break the tree, say, between tips t76 and t43. After some fiddling, we can figure out where that split point needs to be:

> splitplotTree(tree,fsize=0.7,ftype="i",split=0.54)

Note that the vertical position of the tree is automatically adjusted so that the spacing of the tips is equal in both panels.

If we would rather plot our tree in two windows, rather than in side-by-side panels (say, for publication across two journal pages), we can do that too.

> splitplotTree(tree,fsize=0.7,ftype="i",split=0.54, new.window=TRUE)

Panel 1:

and panel 2:

Finally, the function can also be used to create a silly (and, so far as I know, entirely useless) animation of a tree gradually sliding from one side of a plotting window to another. Here, for the Greater Antillean anole tree:



And the code used to do this (minus functions for saving frames to movie file for external viewing):

# code for animation
layout(matrix(c(1,2),1,2))
plotTree(anoletree,fsize=0.7,ftype="i"); layout(1)
for(i in 198:2) splitplotTree(anoletree,split=i/200,fsize=0.7,ftype="i")
layout(matrix(c(2,1),1,2))
plotTree(anoletree,fsize=0.7,ftype="i"); layout(1)

That's it!