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.shp", package="maptools")[1], IDvar="FIPSNO", proj4string=CRS("+proj=longlat +ellps=clrk66"))  xx.sub1 <- subset(xx, xx$FIPSNO < 37010) xx.sub2 <- subset(xx, xx$FIPSNO > 37010)  xx.sub1@data$id <- rownames(xx.sub1@data) xx.sub1.points <- fortify(xx.sub1, region="id") xx.sub1.df = plyr::join(xx.sub1.points, xx.sub1@data, by="id")  xx.sub2@data$id <- rownames(xx.sub2@data) xx.sub2.points <- fortify(xx.sub2, region="id") xx.sub2.df = plyr::join(xx.sub2.points, xx.sub2@data, by="id")  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 = RColorBrewer::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") 

Up to this point everything works as expected and I get a nice map:

What I'd like to change however is to add separate legend for data from xx.sub1.df - since all polygons are just filled with grey I hope it will be one additional entry.

How can I achieve that?

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!