Change axis label position, tick marks and legend in terra plots in R

Question

I am new to using the terra package in R. I am trying to plot a SpatRaster using the package's plot function. However, I would like to change multiple layout characteristics but did not find the needed information online.

For example:

# Example data 
library(terra)
f <- system.file("ex/elev.tif", package="terra")
r <- rast(f)

## Define colours
colfunc <- colorRampPalette(c("#08306b", "#f7fbff"))

# list with details for legend
plg = list(
        title = expression(bold("Depth [m]")),
        title.cex = 0.9,
        cex = 0.7,
        shrink=0
        )

# list with details for axes
pax = list( 
     sides = c(1,2,3,4),
     labels = T
)
   

plot(r, mar=c(3.1, 3.1, 2.1, 7.1), plg=plg, pax=pax, las=1, col=colfunc(20))

This results in the following plot:

Now my question: is it possible to

a) increase the distance between the legend and the axis? b) have tick marks on all 4 axis sides but label the tick marks on 2 sides only? c) create tick marks that contain "longitude and latitude" information, i.e. N and W in it? d) change the position and label of the tick marks?

i.e. something like this:

I am aware of the plg = and pax = arguments within the plot() call but I was not able to find a list of all possible arguments within plg and pax as they are seemingly different from the arguments used in the "normal" axis functions.

Answer 1

a) increase the distance between the legend and the axis?

You can use plg$ext to set the location of a continuous legend (it works a bit different for other legend types and that needs to be improved)

b) have tick marks on all 4 axis sides but label the tick marks on 2 sides only?

You can use pax() values tick=1:4, side=1:4, lab=1:2

c) create tick marks that contain "longitude and latitude" information, i.e. N and W in it?

You can use pax() argument retro=TRUE (should get a better name) to use a sexagesimal notation.

For example

library(terra)
#terra 1.6.24
f <- system.file("ex/elev.tif", package="terra")
r <- rast(f)
colfunc <- colorRampPalette(c("#08306b", "#f7fbff"))

# list with details for legend
plg = list(ext=c(6.75, 6.8, 49.5, 50.1),
        title = expression(bold("Depth [m]")),
        title.cex = 0.9, cex = 0.7, shrink=0 )
        
pax=list(side=2:3, tick=c(1,4), lab=1:2, retro=TRUE)
plot(r, plg=plg, pax=pax, las=1, col=colfunc(20))

d) change the position and label of the tick marks?

You can use pax() arguments xat and yat for the position and xlabs and ylabs for the labels. For example

pax <- list(xat=c(6,6.4), yat=c(49.5, 49.75, 50), ylabs=c("A", "B", "C"), 
            retro=TRUE, tick=1:4)
plot(r, plg=plg, pax=pax, las=1, col=colfunc(20))

The level of control that you were looking for was not available in the CRAN version (in a reasonably easy way), and you effectively requested new features (and they have not been documented yet). The best place to do that is on github, but I understand that it can be hard to know whether something cannot be done (feature request) or whether you just do not know how (ask for help on SO).