Adding gridlines to a phylo.to.map plot

A phytools blog reader asks:

“Is it possible to add latitude and longitude tick marks with this function phylo.to.map?”

The answer is - yes. Well, at least we can adjust the margins to include x & y axes (which are on the scale of lat. & long.), and (if we want to) we can even overlay lines of latitude & longitude on the plot. Here's how:

## first simulate a tree & some data:
library(phytools)
tree<-pbtree(n=26,scale=100,tip.label=LETTERS)
lat<-fastBM(tree,sig2=10,bounds=c(-36,-16),a=-20)
long<-fastBM(tree,sig2=80,bounds=c(115,150),a=130)
loc<-cbind(lat,long)
loc

##         lat     long
## A -30.00298 136.8887
## B -29.52615 139.2913
## C -21.94540 117.6450
## D -21.44360 137.3063
## E -30.69070 129.6878
## F -27.91373 138.7563
## G -21.24415 119.6728
## H -19.85676 138.0375
## I -29.70913 143.8215
## J -32.41438 130.5098
## K -34.28836 127.8772
## L -32.97290 135.9813
## M -26.05441 145.2799
## N -23.89187 124.5674
## O -17.84134 140.5776
## P -28.67361 138.5719
## Q -16.77305 128.0718
## R -26.44700 140.7354
## S -24.02262 139.6478
## T -35.53304 136.3906
## U -30.89431 139.9094
## V -19.50736 149.8240
## W -21.92831 149.1358
## X -18.30595 138.6660
## Y -19.17043 140.1686
## Z -22.59701 132.3479

Now, with these (made-up) data in hand, let's create our plot & customize it:

## now plot projection
phylo.to.map(tree,loc,ylim=c(-40,-10), xlim=c(110,155),
    mar=c(5.1,4.1,2.1,2.1))

## objective: 180
## objective: 172
## objective: 172
## objective: 172
## objective: 168
## objective: 168
## objective: 168
## objective: 154
## objective: 140
## objective: 140
## objective: 140
## objective: 140
## objective: 140
## objective: 138
## objective: 138
## objective: 134
## objective: 134
## objective: 134
## objective: 134
## objective: 134
## objective: 134
## objective: 126
## objective: 126
## objective: 126
## objective: 126

## create the axes
long.lines<-seq(110,160,by=10)
lat.lines<-seq(-40,-10,by=10)
axis(1,at=long.lines)
axis(2,at=seq(-40,-10,by=5))
## add gridlines
for(i in 1:length(lat.lines))
    lines(c(par()$usr[1],160),rep(lat.lines[i],2),lty="dotted",
        col="#00000040")
for(i in 1:length(long.lines))
    lines(rep(long.lines[i],2),c(par()$usr[3],-10),lty="dotted",
        col="#00000040")
lines(c(par()$usr[1],160),rep(-23,43333,2),lwd=2,col="#00000080")
text(105,-23.5,"Tropic of Capricorn",cex=0.7,pos=4)
title(xlab="longitude")
title(ylab="latitude                         ")

Well, you get the idea.