The lineLabel function produces and draws text grobs following the paths defined by a list of Line objects. The sp.lineLabel methods use this function to work easily with spplot. Let’s use the meuse data to illustrate how to use it:

data(meuse.grid)
coordinates(meuse.grid) = ~x+y
proj4string(meuse.grid) <- CRS("+init=epsg:28992")
gridded(meuse.grid) = TRUE


data(meuse)
coordinates(meuse) = ~x+y
data(meuse.riv)
## Create a SpatialLines object with ID=1
meuse.sl <- SpatialLines(list(Lines(list(Line(meuse.riv)), "1")))

library(RColorBrewer)
myCols <- adjustcolor(colorRampPalette(brewer.pal(n=9, 'Reds'))(100), .85)

## label is a wrapper to build a character vector with names
## according to the SpatialLines ID's
labs <- label(meuse.sl, 'Meuse River')

With textloc you can choose where the label must be located. For example, with maxDepth the label will start at the point where the line reaches its maximum depth:

sl1 <- list('sp.lineLabel', meuse.sl, label=labs,
            position='below', textloc='maxDepth',
            spar=.2,
            col='darkblue', cex=1,
            fontfamily='Palatino',
            fontface=2)

spplot(meuse.grid["dist"],
       col.regions=myCols, 
       sp.layout = sl1)

The default is to place the label in a stable region where the sign of the slope remains constant.

sl2 <- modifyList(sl1, list(textloc = 'constantSlope'))

spplot(meuse.grid["dist"],
       col.regions=myCols, 
       sp.layout = sl2)

But you can define the label location with a numeric index relative to the line length:

sl3 <- modifyList(sl1, list(textloc = 140, position='above'))

spplot(meuse.grid["dist"],
       col.regions=myCols, 
       sp.layout = sl3)

There is a more sophisticated example in the Spatial Data chapter of my forthcoming book.

This package already provided the pointLabel function with optimization routines to find good locations for point labels without overlaps. This function, useful for base graphics, has been adapted to work with lattice graphics and with the spplot functions.

You will find the panel.pointLabel to use with xyplot and the rest of the lattice family:

library(maptools)
library(lattice)

n <- 15
x <- rnorm(n)*10
y <- rnorm(n)*10
labels <- as.character(round(x, 5))


myTheme <- list(add.text=list(
                  cex=0.7,
                  col='midnightblue',
                  fontface=2,
                  fontfamily='mono'))

xyplot(y~x,
       labels=labels,
       par.settings=myTheme, 
       panel=function(x, y, labels, ...){
         panel.xyplot(x, y, ...)
         panel.pointLabel(x, y, labels=labels, ...)
       })

And there is the sp.pointLabel method to be combined with spplot:

data(meuse.grid)
coordinates(meuse.grid) = ~x+y
proj4string(meuse.grid) <- CRS("+init=epsg:28992")
gridded(meuse.grid) = TRUE

library(RColorBrewer)
myCols <- adjustcolor(colorRampPalette(brewer.pal(n=9, 'Reds'))(100), .85)

pts <- spsample(meuse.grid, n=15, type="random")

## Let's print the name of some R authors 
Rauthors <- readLines(file.path(R.home("doc"), "AUTHORS"))[9:28]
someAuthors <- Rauthors[seq_along(pts)]

sl1 <- list('sp.points', pts, pch=19, cex=.8, col='midnightblue')
sl2 <- list('sp.pointLabel', pts, label=someAuthors,
            cex=0.7, col='midnightblue',
            fontfamily='Palatino')

spplot(meuse.grid["dist"], col.regions=myCols, sp.layout=list(sl1, sl2))

There is a more sophisticated example in the Spatial Data chapter of my forthcoming book.