Plotting a legend outside the plot area using add.color.bar (& in general)

Recently I received the following question:

“I am using the add.color.bar function to add a colorbar to an existing plot.
Do you know if it is possible to move the added colorbar to the outside of the plot domain? Do you think I would need to modify code in the actual function?”

Evidently, this can be done using par(xpd=TRUE). For instance:

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 
##  3.09148522  1.54096027  2.91508379  2.00469068  1.15813551  0.29543463 
##           G           H           I           J           K           L 
##  0.27951056  0.60805259 -2.52405841 -3.50891295 -3.34674083 -0.97428407 
##           M           N           O           P           Q           R 
## -1.25336677 -1.40812351 -0.51371266 -2.92582374 -2.24608676 -0.53203275 
##           S           T           U           V           W           X 
## -0.59487179  0.30811419 -0.08364322  0.16895725  0.70189973 -3.64724619 
##           Y           Z 
## -3.65517946 -4.30271550

obj<-contMap(tree,x,plot=FALSE)
plot(obj,legend=FALSE,mar=c(5.1,2.1,2.1,2.1))
par(xpd=TRUE)
add.color.bar(leg=3,cols=obj$cols,title="trait value",lims=obj$lims,
    digits=3,prompt=FALSE,x=0,y=-3)

We can easily see that this is outside of our plotting domain by adding axis labels to the plot as follows:

plot(obj,legend=FALSE,mar=c(6.1,2.1,2.1,2.1))
par(xpd=TRUE)
add.color.bar(leg=3,cols=obj$cols,title="trait value",lims=obj$lims,
    digits=3,prompt=FALSE,x=0,y=-4)
axis(1)

You get the general idea.