library(mapdata)
data(us.cities)

map("state", col="gray90", fill=TRUE, lwd=0.1, mar=c(0,0,0,0), xlim=c(-127,-65))
map("worldHires", "Canada", xlim=c(-127,-110), ylim=c(48,64), col="gray95", fill=TRUE, add=TRUE) 
map("worldHires","Mexico", xlim=c(-127,-110), ylim=c(28,45), col="gray95", fill=TRUE, add=TRUE) 
points(gt$long, gt$lat, cex=pt_scale, pch=19, col=MAP_BLACK)

map.cities(us.cities, minpop=5e5, label=TRUE, pch=19, col='red')
title("Locations of Github Users", line=1)