library(maps)

map('county', 'ca', interior=TRUE)
map.scale()
map.axes()

## add some user data in lon/lat format:
points(x, pch=4, cex=0.5, col=1)
points(y, pch=4, cex=0.5, col=2)
points(z, pch=4, cex=0.5, col=3)