Plotting bar charts on map using ggplot2?

I would like to produce a barplot for each location on a map using ggplot2, as was done by xingmowang with base graphics and a number of packages:

http://nzprimarysectortrade.wordpress.com/2011/10/02/let-r-fly-visualizing-export-data-using-r/

This is related to Embedding a miniature plot within a plot.

For the moment, the best I can do is match value to point size in a jittered point plot:

require(ggplot2)
require(maps)

#Get world map info
world_map <- map_data("world")

#Creat a base plot
p <- ggplot() + coord_fixed()

#Add map to base plot
base_world <- p + geom_polygon(data=world_map,
                               aes(x=long,
                                   y=lat,
                                   group=group))


#Create example data
geo_data <- data.frame(long=c(20,20,100,100,20,20,100,100),
                       lat=c(0,0,0,0,0,0,0,0),
                       value=c(10,30,40,50,20,20,100,100),
                       Facet=rep(c("Facet_1", "Facet_2"), 4),
                       colour=rep(c("colour_1", "colour_2"), each=4))

#Creat an example plot
map_with_jitter <- base_world+geom_point(data=geo_data,
                                            aes(x=long,
                                                y=lat,
                                                colour=colour,
                                                size=value),
                                         position="jitter",
                                         alpha=I(0.5))

#Add faceting
map_with_jitter <- map_with_jitter + facet_wrap(~Facet)

map_with_jitter <- map_with_jitter + theme(legend.position="none")