How to insert caption into ggplot maps?

Question

How do I insert the legend and scale the map to degrees?

library("rgeos")
library("maptools")
library("ggplot2")
library("mapproj")

cores1 = c('brown2', 'aquamarine2', 'darkgray','darkolivegreen1', 'floralwhite')

ggplot() + geom_polygon(data=mapa, mapping=aes(x=long, y=lat, group=group), 
                        color="black", fill=cores1[mapa$tb]) + 
           coord_map() + 
           labs(y="latitude", x="longitude") +
           theme(plot.title=element_text(hjust = 5)) 

Answer 1

The ggplot is not including the legend because you are assigning colors as attribute and not as mapping. |Set attributes does not include new information in the chart (map), only changes visual aspects. For this reason do not need caption.

Already map a variable in some aesthetic aspect (within the aes(...) is to add information to the chart (map), for this information to be decoded, it needs caption.

By simply including the fill = tb within the aes(...), ggplot itself already creates the caption.

Recreating the data

I used this object mapa to build as maps below:

mapa <- map_data("state")
mapa$tb <- factor(sample(1:5, size = nrow(mapa), replace = TRUE))

Plotting the maps

By simply removing the fill=cores1[mapa$tb] and include fill = tb within the aes()

ggplot() + 
  geom_polygon(data=mapa, mapping=aes(x=long, y=lat, group=group, fill = tb),
               color="black") + 
  coord_map() + 
  labs(y="latitude", x="longitude") +
  theme(plot.title=element_text(hjust = 5))

But in this solution we lost the colors you wanted. To repeat them with the proposed methodology, we have to add a manual color scale with the function scale_fill_manual(). The argument that will be passed to this function is precisely the vector with the colors you chose. The argument valuesshould be explicit. So we have:

ggplot() + 
  geom_polygon(data=mapa, mapping=aes(x=long, y=lat, group=group, fill = tb),
               color="black") + 
  coord_map() + 
  labs(y="latitude", x="longitude") +
  theme(plot.title=element_text(hjust = 5)) +
  scale_fill_manual(values = cores1)