world_data <- map_data("world")

ggplot() + scale_y_continuous(limits=c(-90,90), expand=c(0,0)) +
scale_x_continuous(expand=c(0,0)) +
theme(axis.ticks=element_blank(), axis.title=element_blank(),
      axis.text=element_blank()) +
geom_polygon(data=world_data, mapping=aes(x=long, y=lat, group=group), fill='grey')

library(maptools)
data(wrld_simpl)

ggplot() +
  geom_polygon(data=wrld_simpl,
             aes(x=long, y=lat, group=group), fill='grey20') +
  coord_cartesian(xlim=c(-180,180), ylim=c(-90,90)) +
  scale_x_continuous(breaks=seq(-180,180,20)) +
  scale_y_continuous(breaks=seq(-90,90,10))