In ggplot2, how can I add additional legend?

Question

I\'m trying to build a map in ggplot2 using data from separate data frames.

library(maptools)

xx <- readShapePoly(system.file(\"shapes/sids.         


        

Answer 1

I'm not 100% sure this is what you want, but here's how I'd approach the problem as I understand it. If we map some unused geom with any data from xx.sub1.df, but make it invisible on the plot, we can still get a legend for that geom. Here I've used geom_point, but you could make it others.

p <- ggplot(xx.sub2.df) + 
  aes(long, lat, fill = (SID79/BIR79)*1000, group = group) + 
  geom_polygon() + geom_path(color="grey80") +
  coord_equal() + 
  scale_fill_gradientn(colours = brewer.pal(7, "YlOrBr")) +
  geom_polygon(data = xx.sub1.df, fill = "grey50") + 
  geom_path(data = xx.sub1.df, color="grey80") +
  labs(fill = "Mapped value", title = "Title")

#Now we add geom_point() setting shape as NA, but the colour as "grey50", so the 
#legend will be displaying the right colour

p2 <- p + geom_point(data = xx.sub1.df, aes(size="xx.sub1", shape = NA), colour = "grey50")

Now we just need to alter the size and shape of the point on the legend, and change the name of the legend (thanks to @DizisElferts who demonstrated this earlier).

p2 + guides(size=guide_legend("Source", override.aes=list(shape=15, size = 10)))

Of course you can change the way the labels work or whatever to highlight what you want to show.

If this isn't what you're after, let me know!