r - Specifying the colour scale for maps in ggplot

Question

Having difficulty setting the colour scales for maps in ggplot. I need greyscale. Very grateful for an idea where I'm going wrong. I also wonder if there is a more efficient way of getting the colour variable into ggplot (i.e. than by attaching it to 'fortified' data)?

library(ggplot2)
states <- map_data("state")
var <- data.frame(table(states$region)) # using rows as a dummy variable
states$variable <- var$Freq[match(states$region,var$Var1)]

map <- ggplot(states, aes(x=long, y=lat)) + 
  geom_polygon(aes(group=group, fill=variable), col=NA,lwd=0)

map + scale_colour_gradient(low='white', high='grey20')
map + scale_colour_grey()

Answer 1

You need to use scale_fill_* and not scale_color_*. For the polygon geometry the fill color of the polygons is coupled to the fill aesthetic, not to the color aesthetic. In general, the function used to change the details of a particular scale is scale_aesthetic_name_type_of_scale, e.g. scale_fill_gradient.