Labeling Extreme Legends in a Map with R and ggplot2

Introduction

In this tutorial, we will explore how to label extreme legends in a map using the popular data visualization library ggplot2 in R. We will use the example of plotting a coefficient number for each state of Argentina and labeling the highest values as “Similar Income” and the lowest as “Different Income”. The process involves modifying the existing code to add custom labels to the legend, which can be achieved using the guide argument within the scale_fill_gradient() function.

Understanding ggplot2

Before diving into the solution, it’s essential to understand how ggplot2 works. The library provides a grammar-based approach to creating visualizations, where each layer is composed of several components: data, aesthetic mapping, and geometry. In this example, we have a simple bar chart with a gradient fill. The key components are:

• Data: The shapefile (nc) containing the data for Argentina’s states.
• Aesthetic mapping: We use aes() to map the AREA column in the dataset to the fill aesthetic.
• Geometry: The geom_sf() function is used to create a geometric shape (in this case, a polygon) that represents each state on the map.

Modifying the Existing Code

To add custom labels to the legend, we need to modify the existing code. Here’s an updated version of the original code:

library(ggplot2)
library(sf)

# Load data
nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)

# Create a bar chart with a gradient fill and custom labels
ggplot(nc) +
    geom_sf(aes(fill = AREA)) +
    scale_fill_gradient(low = "green",
                        high = "green4",
                        breaks = range(nc$AREA),
                        guide = "legend",
                        labels = c("Similar Income", "Different Income"))

In this updated code, we added the guide argument within the scale_fill_gradient() function. This argument specifies the type of legend to be displayed and allows us to customize its appearance.

Understanding the Guide Argument

The guide argument in ggplot2 is used to specify the type of legend to be displayed. In our example, we chose "legend" as the guide type, which means we will display a standard legend with numerical labels.

However, if we wanted to add custom labels like “Similar Income” and “Different Income”, we can use the labels argument within the scale_fill_gradient() function. This argument takes a character vector containing the labels for each color in the gradient.

Using Custom Labels

To label our extreme legends with custom text, we need to modify the existing code. Here’s an updated version of the original code:

library(ggplot2)
library(sf)

# Load data
nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)

# Create a bar chart with a gradient fill and custom labels
ggplot(nc) +
    geom_sf(aes(fill = AREA)) +
    scale_fill_gradient(low = "green",
                        high = "green4",
                        breaks = range(nc$AREA),
                        guide = "legend",
                        labels = c("Similar Income", "Different Income"))

However, this code will not work as expected. The reason is that scale_fill_gradient() does not support custom labels for each color in the gradient.

Solved!

To solve our problem, we need to use a different approach. We can create two separate scale specifications with their respective legend labels and then combine them using the guide argument within the scale_fill_gradient() function.

Here’s an updated version of the original code:

library(ggplot2)
library(sf)

# Load data
nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)

# Create a bar chart with a gradient fill and custom labels
ggplot(nc) +
    geom_sf(aes(fill = AREA)) +
    scale_fill_gradient(low = "green",
                        high = "green4") +
    scale_fill_manual(values = c("Similar Income" = green4, "Different Income" = green),
                      labels = c("Similar Income", "Different Income")) +
    theme(legend.position = c(0.8, 0.5))

However, this code will still not work as expected. The reason is that scale_fill_manual() does not support the same aesthetics as scale_fill_gradient(). This means we need to use a different approach.

A Different Approach

To add custom labels to our legend, we can create two separate scale specifications with their respective legend labels and then combine them using a custom guide. Here’s an updated version of the original code:

library(ggplot2)
library(sf)

# Load data
nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)

# Create a bar chart with two separate scale specifications and a custom guide
ggplot(nc) +
    geom_sf(aes(fill = AREA)) +
    scale_fill_gradient(low = "green",
                        high = "green4") +
    scale_fill_manual(values = c("Similar Income" = green4, "Different Income" = green),
                      labels = c("Similar Income", "Different Income")) +
    scale_fill_brewer(labels = c("Similar Income", "Different Income")) +
    theme(legend.position = c(0.8, 0.5)) +
    guides(fill = guide_legend(override.aes = list(label = c("Similar Income", "Different Income"))))

However, this code will still not work as expected. The reason is that scale_fill_brewer() does not support custom labels.

Final Solution

To add custom labels to our legend, we can use a combination of scale_fill_gradient() and guide_legend(). Here’s an updated version of the original code:

library(ggplot2)
library(sf)

# Load data
nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)

# Create a bar chart with a gradient fill and custom legend labels
ggplot(nc) +
    geom_sf(aes(fill = AREA)) +
    scale_fill_gradient(low = c(0, 1),
                        high = c(1, 0),
                        mid = c(1/2, 1/2), # create a mid point that splits the gradient into two equal parts
                        values = c("Similar Income", "Different Income"),
                        labels = c("Similar Income", "Different Income")) +
    theme(legend.position = c(0.8, 0.5))

In this final solution, we used scale_fill_gradient() with custom values to split the gradient into two equal parts and created a mid point that splits the gradient into two equal parts.

We also used labels argument within the scale_fill_gradient() function to specify the labels for each color in the gradient.

Finally, we used theme(legend.position = c(0.8, 0.5)) to position the legend at the specified location.

I hope this helps!

Last modified on 2024-11-05