A cleaned up continuous solution for: http://stackoverflow.com/questions/24136868/plot-map-with-values-for-countries-as-color-in-r

map-fill.R

library(RColorBrewer)
library(maptools)
library(ggplot2)
library(mapproj)
library(rgdal)

## make sure you call
gpclibPermit() 

data(wrld_simpl)
ddf = read.table(text="
                 country value
                 'United States' 10
                 'United Kingdom' 30
                 'Sweden' 50
                 'Japan' 70
                 'China' 90
                 'Germany' 100
                 'France' 80
                 'Italy' 60
                 'Nepal' 40
                 'Nigeria' 20", header=TRUE)

pal <- colorRampPalette(brewer.pal(9, 'Reds'))(length(ddf$value))
palSz <- 10 # Adjust for your desired range

wrld_simpl@data$id <- wrld_simpl@data$NAME
wrld <- fortify(wrld_simpl, region="id")
wrld <- subset(wrld, id != "Antarctica") # we don't rly need Antarctica

## start a blank plot
gg <- ggplot()

## setup base world map
gg <- gg + geom_map(data=wrld,
                    map=wrld,
                    aes(map_id=id, x=long, y=lat),
                    fill="white",
                    color="#7f7f7f",
                    size=0.25)

gg <- gg + geom_map(data=ddf,
                    map=wrld,
                    aes(map_id=country, fill=value),
                    color="white",
                    size=0.25)

gg <- gg + scale_fill_gradient2(low = pal[1],
                                mid = pal[palSz/2],
                                high = pal[palSz],
                                midpoint = (max(ddf$value) + min(ddf$value)) / 2,
                                name="value")

## this gives us proper coords. mercator proj is default
gg <- gg + coord_map()
gg <- gg + labs(x="", y="")

## if you want to hide the tick marks and background
gg <- gg + theme(plot.background = element_rect(fill = "transparent", colour = NA),
                  panel.border = element_blank(),
                  panel.background = element_rect(fill = "transparent", colour = NA),
                  panel.grid = element_blank(),
                  axis.text = element_blank(),
                  axis.ticks = element_blank(),
                  legend.position = "right")
(gg)