levelplot_custom.R

# adding a custom legend to levelplot when z values are categorical (not continuous)
library(lattice)
foo <- expand.grid(x = 1:10, y=1:10)
foo$z <- as.integer(runif(100)*3)
# foo$z takes on values of 0, 1 or 2: this could represent a factor

# default levelplot
levelplot(z~x*y, foo)

# levelplot with custom colors and custom legend
my.colors <- c("blue", "green", "red")
my.labels <- c("category 1", "category 2", "category 3")
levelplot(z~x*y, foo,
          col.regions = my.colors,
          cuts = 2,
          colorkey=FALSE,
          key=list(space="top", columns=3,
                   rect=list(col=my.colors, border="transparent", size=1),
                   text=list(lab=my.labels),
                   cex=0.6,
                   between.columns=3,
                   between=1)
) 

# levelplot with custom colors and custom legend on right side
levelplot(z~x*y, foo,
          col.regions = my.colors,
          cuts = 2,
          colorkey=FALSE,
          key=list(space="right", columns=1,
                   rect=list(col=my.colors, border="transparent", size=2, height=0.6),
                   text=list(lab=my.labels),
                   cex=0.6,
                   between.columns=3,
                   between=1,
                   padding.text=4)
) 

# see also: 
# http://www.magesblog.com/2012/12/changing-colours-and-legends-in-lattice.html#more
# https://stat.ethz.ch/pipermail/r-sig-geo/2014-September/021734.html