Using a fixed color map for two or more plotted "contMap" objects

A phytools user recently submitted the following request:

“I'm writing to ask for help to make a plot for two trait with contMap function, and use a single legend for the two traits. The traits have the same unit, but the ranges of the values are different. When I make a plot of traits, the colors did not represent the same values (I think) and I would like to show into a unique legend the values for the two traits.
Can you help me. please?”

This is fairly straightforward to do using the argument lims as follows.

Let's imagine a phylogeny, tree, and two characters, x & y:

library(phytools)
tree

## 
## Phylogenetic tree with 26 tips and 25 internal nodes.
## 
## Tip labels:
##  A, B, C, D, E, F, ...
## 
## Rooted; includes branch lengths.

x

##           A           B           C           D           E           F 
## -1.47498137 -0.61091402 -0.01033534  0.06370485 -0.59229611 -0.37826068 
##           G           H           I           J           K           L 
## -1.79697655 -1.84468301  0.42588201  0.26821021 -0.78436474 -1.15474354 
##           M           N           O           P           Q           R 
## -1.26065197 -1.21052577 -0.81250890 -0.83428720 -1.66350965 -0.30785186 
##           S           T           U           V           W           X 
##  0.17551957  0.03223967 -0.14868392  0.05674916  0.29106350  0.04371598 
##           Y           Z 
##  0.03394977  0.31722898

y

##          A          B          C          D          E          F 
## 2.62095178 2.34889317 0.98804069 1.76638374 1.61583210 1.73510014 
##          G          H          I          J          K          L 
## 1.16068504 1.29077801 0.52935367 2.00890012 2.96501063 2.37973152 
##          M          N          O          P          Q          R 
## 2.28330800 1.95555508 2.04026749 2.33749236 0.84591649 2.00535665 
##          S          T          U          V          W          X 
## 2.16052968 2.74378292 1.63802572 0.09176915 0.64961048 0.35952349 
##          Y          Z 
## 0.82434655 0.74167574

The user is right that if these two characters were plotted separately using the function contMap they would indeed use different color maps, determined by the range of values in each vector. For instance:

par(mfrow=c(1,2))
plot(setMap(contMap(tree,x,plot=F),col=c("black","white")),lwd=6,legend=0.8,
    xlim=c(-0.1,1.1))
plot(setMap(contMap(tree,y,plot=F),col=c("black","white")),lwd=6,legend=0.8,
    xlim=c(-0.1,1.1))

To force them to have the same color maps, we just need to set the argument lims to be a constant as follows:

lims<-range(c(x,y))
m1<-contMap(tree,x,plot=F,lims=lims)
m1

## Object of class "contMap" containing:
## 
## (1) A phylogenetic tree with 26 tips and 25 internal nodes.
## 
## (2) A mapped continuous trait on the range (-1.844683, 2.965011).

m2<-contMap(tree,y,plot=F,lims=lims)
m2

## Object of class "contMap" containing:
## 
## (1) A phylogenetic tree with 26 tips and 25 internal nodes.
## 
## (2) A mapped continuous trait on the range (-1.844683, 2.965011).

and we can plot them however we choose:

par(mfrow=c(1,2))
plot(setMap(m1,col=c("black","white")),lwd=6,legend=0.8,
    xlim=c(-0.1,1.1))
plot(setMap(m2,col=c("black","white")),lwd=6,legend=0.8,
    xlim=c(-0.1,1.1))

To avoid repeating the legend, we can just do this:

par(mfrow=c(1,2))
plot(setMap(m1,col=c("black","white")),lwd=6,legend=1,
    xlim=c(-0.1,1.1))
ylim<-get("last_plot.phylo",envir=.PlotPhyloEnv)$y.lim
plot(setMap(m2,col=c("black","white")),lwd=6,legend=F,ylim=ylim,
    xlim=c(-0.1,1.1))

All there is to it.