ggplot2 Cheat Sheet:
Three Variables

library(ggplot2)

Contour Map

seals$z <- with(seals, sqrt(delta_long^2 + delta_lat^2))
p <- ggplot(seals, aes(long, lat))
p <- p + geom_contour(aes(z = z))
print(p)

Raster Map

seals$z <- with(seals, sqrt(delta_long^2 + delta_lat^2))
p <- ggplot(seals, aes(long, lat))
p <- p + geom_raster(aes(fill = z))
print(p)

Tile Map

seals$z <- with(seals, sqrt(delta_long^2 + delta_lat^2))
p <- ggplot(seals, aes(long, lat))
p <- p + geom_tile(aes(fill = z))
print(p)