ggplot2 Cheat Sheet:
Two Variables: Maps

library(ggplot2)

Map Plot

data <- data.frame(
    murder = USArrests$Murder, state = tolower(rownames(USArrests))
)
map <- map_data("state")
p <- ggplot(data, aes(fill = murder))
p <- p + geom_map(aes(map_id = state), map = map)
p <- p + expand_limits(x = map$long, y = map$lat)
p <- p + ylim(22, 53)
print(p)