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.
Map
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

What do you want your legend to show? To create legends with ggplot2 `fill` and/or `color` should be include within `aes`. You might find useful the following link: https://www.r-spatial.org/r/2018/10/25/ggplot2-sf.html — davidnortes, Jun 30 '20 at 20:07
Hi davidnortes, thank you for your response! I'm trying to include in the legend the purple points and the green points. Thank you for providing that link, and I was using it quite a bit, but unfortunately I'm not fully understand how to use prairie to get that legend. — KelseyB, Jun 30 '20 at 20:11
Hi davidnortes, thank you for your help as I was able to fix my mistake! I was using aes in the wrong line of the code and not using color properly — KelseyB, Jun 30 '20 at 20:57

davidnortes · Answer 1 · 2020-06-30T21:06:44.303

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")

Hi davidnortes, yes, thank you so much for your help :) seeing the code helped me find my error as I was putting the aes function in the wrong line — KelseyB, Jun 30 '20 at 21:03

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

1 Answers1