Another technique for including a time axis in a circular fan-style tree

In a prior post I showed how to add a time axis to a plotted fan tree using the plotTree / plot.simmap argument part.

Though this works well, our open angle is between part × 360^o and 360^o, rather than centered around 0^o / 360⁰. To do this, we should ignore the argument part & instead combine the poorly documented arguments tips and maxY. These arguments exist to allow the user to supply a custom tip height spacing/order for a square phylogram; however, we used in a fan-style tree the effect is to transform these values into the circular space.

Here's what I mean:

library(phytools)
library(plotrix)
tree

## 
## Phylogenetic tree with 400 tips and 399 internal nodes.
## 
## Tip labels:
##  t307, t308, t181, t128, t129, t110, ...
## 
## Rooted; includes branch lengths.

plotTree(tree,type="fan",tips=seq(5,380,by=375/399),maxY=400,
    ftype="off",lwd=1)
T<-max(nodeHeights(tree))
tick.spacing<-10
min.tick<-10
obj<-axis(1,pos=0,at=seq(T,min.tick,by=-tick.spacing),cex.axis=0.5,
    labels=FALSE)

for(i in 1:length(obj)){
    a1<-0
    a2<-2*pi
    draw.arc(0,0,radius=obj[i],a1,a2,lwd=1,
        col=make.transparent("blue",0.1))
}

axis(1,pos=0,at=seq(T,min.tick,by=-tick.spacing),cex.axis=0.5,
    labels=FALSE)
text(obj,rep(-5,length(obj)),T-obj,cex=0.6)
text(mean(obj),-10,"time (mybp)",cex=0.8)

Neat.

Note that in theory we could do this with the ape function axisPhylo, although this function seems to be somewhat unpredictable as to whether or not it works as expected. For instance:

plotTree(tree,type="fan",tips=seq(15,385,by=370/399),maxY=400,
    ftype="off",lwd=1)
axisPhylo()