How do I add a legend using aes, ggplot2 and maps?

Question

I'm trying to make a map with points plotted for the Canadian prairie provinces, but I'm having no luck adding in a legend to my map. I'm very new to mapping in r, so I'm not understanding how I should include aes to get a legend. My data for siteDataTrees is from an excel csv file and the top looks like this: siteDataTrees and the data for siteDataBoth is also from a csv file and the top looks like this: siteDataBoth.

Here's what I have so far for my code:

library(maps)
library(ggplot2)
library(sf)

prairies1 <- map("worldHires","Canada", xlim = c(-120,-87), ylim = c(49,61),
             plot = FALSE, fill = TRUE)
prairies <- st_as_sf(prairies1)

ggplot(data = prairies) +  
    geom_sf() +
    geom_point(data = siteDataTrees, aes(x = long, y = lat), size = 2.5, pch = 21, 
     fill = "purple", show.legend = TRUE) +
    geom_point(data = siteDataBoth, aes(x = long, y = lat), size = 2.5, pch = 21, 
     fill = "light green", show.legend = TRUE) +
    geom_text(data = locations, aes(x = long, y = lat, label = name), 
            size = 2, col = "black", check_overlap = FALSE) +
    annotation_scale(location = "tr", width_hint = 0.2) +
    ggtitle("Climate Stations and Tree Chronology Locations for South AB") +
    labs(x = "latitude", y = "longitude") +
    theme(legend.position = "right") + 
    coord_sf(xlim = c(-115, -110), ylim = c(48.9, 50.49), expand = FALSE)

I've also included a map to show what it looks like without the legend.

How should I take the data frame prairies and use it with aes to include a legend? Is there another way to add in a legend in ggplot2 without using the aes function? Thank you in advance for your help and please let me know if something is missing as this is my first posting

Answer 1

Let me give you a couple of examples on how to work out a legend using a slightly modified example from r-spatial.

First we prepare the data:

library(maps)
library(ggplot2)
library(sf)
library(rnaturalearth)
library(rnaturalearthdata)

world <- ne_countries(scale = "medium", returnclass = "sf")
(sites <- data.frame(longitude = c(-80.144005, -80.109), 
                     latitude = c(26.479005,26.83), 
                     type = c("tree", "station")))

Now we plot. Case 1: color is not an issue

ggplot(data = world) +
  geom_sf() +
  geom_point(data = sites, 
             aes(x = longitude, y = latitude, fill = type), 
             size = 4, 
             shape = 23) +
  coord_sf(xlim = c(-88, -78), ylim = c(24.5, 33), expand = FALSE) +
  theme(legend.position = "bottom")

Case 2: fill color is an issue. Here we can use a named vectors to pass the colors and the labels we want by type of point. For example:

mapfill <- c('tree' = "forestgreen", 'station' = "purple")
maplab <- c('tree' = "trees in prairies", 'station' = "Stations in prairies")

Then we plot combining both mapfill and maplab:

ggplot(data = world) +
  geom_sf() +
  geom_point(data = sites, aes(x = longitude, y = latitude, fill = type), size = 4, 
             shape = 23) +
  scale_fill_manual(values = mapfill, labels = maplab) +
  coord_sf(xlim = c(-88, -78), ylim = c(24.5, 33), expand = FALSE) +
  theme(legend.position = "bottom")

Remark 1 If you do not want type in the legend title you can either delete it using legend. title = element_blank() within theme

Remark 2 If, instead of fill, you are using color, use function scale_color_manual. If you are combining both fill and color do the same with scale_***_manual

In spirit of full disclosure, if you do not mind the colors and you want a quick fix (and I cannot stress this enough) you can also code fill = "TextYouWantInLegend" within aes. See following example:

ggplot(data = world) +
  geom_sf() +
  geom_point(data = sites[1,], aes(x = longitude, y = latitude, fill = "toto"), size = 4, 
             shape = 23) +
  geom_point(data = sites[2,], aes(x = longitude, y = latitude, fill = "koko"), size = 4, 
             shape = 23) +
  coord_sf(xlim = c(-88, -78), ylim = c(24.5, 33), expand = FALSE) +
  theme(legend.position = "bottom")