r - In ggplot2, how can I add additional legend? -

i'm trying build map in ggplot2 using data 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$id <- rownames(xx.sub1@data) xx.sub1.points <- fortify(xx.sub1, region="id") xx.sub1.df = join(xx.sub1.points, xx.sub1@data, by="id")  xx.sub2@data$id <- rownames(xx.sub2@data) xx.sub2.points <- fortify(xx.sub2, region="id") xx.sub2.df = 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 = 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 point works expected , nice map:

what i'd change add separate legend data xx.sub1.df - since polygons filled grey hope 1 additional entry.

how can achieve that?

i'm not 100% sure want, here's how i'd approach problem understand it. if map unused geom data xx.sub1.df, make invisible on plot, can still legend geom. here i've used geom_point, make 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 add geom_point() setting shape na, colour "grey50",  #legend displaying right colour  p2 <- p + geom_point(data = xx.sub1.df, aes(size="xx.sub1", shape = na), colour = "grey50") 

now need alter size , shape of point on legend, , change name of legend (thanks @diziselferts demonstrated earlier).

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

of course can change way labels work or whatever highlight want show.

if isn't you're after, let me know!