Creating Custom Legends from Scratch with ggplot2: A Step-by-Step Guide to Customizing Color Maps and Adding Labels

Creating a Custom Legend from Scratch in ggplot2

Creating a custom legend in ggplot2 is an essential skill for any data visualization enthusiast. In this article, we will explore how to create a custom legend using ggplot2’s scale_colour_manual function.

Introduction

ggplot2 is a powerful and flexible data visualization library written in R. One of its key features is the ability to customize the appearance of plots, including legends. Legends allow us to associate different colors with specific variables or categories in our data. In this article, we will show how to create a custom legend using ggplot2’s scale_colour_manual function.

Creating a Custom Color Map

The first step in creating a custom legend is to assign names to each color value in the color map. This allows us to associate each color with a specific variable or category.

# Create a custom color map
my.cols <- rainbow(3)

# Assign names to each color value
names(my.cols) <- c("CI 99", "CI 95", "CI 90")

Using `scale_colour_manual`

The scale_colour_manual function is used to customize the color map in ggplot2. It allows us to specify a vector of colors, with each color associated with a specific variable or category.

# Create a new data frame for our plot
foo <- tibble(name = letters[1:2],
              ci90lwr = c(0.8, 0.9),
              ci90upr = c(1.2, 1.3),
              ci95lwr = c(0.7, 0.8),
              ci95upr = c(1.3, 1.4),
              ci99lwr = c(0.6, 0.7),
              ci99upr = c(1.4, 1.5))

# Create a custom legend using scale_colour_manual
foo %>%
  ggplot() +
    geom_segment(mapping = aes(x= name,
                               xend = name,
                               y= ci90lwr,
                               yend = ci90upr),
                 size=5,col=my.cols[3]) +
    geom_segment(mapping = aes(x= name,
                               xend = name,
                               y= ci95lwr,
                               yend = ci95upr),
                 size=3,col=my.cols[2]) +
    geom_segment(mapping = aes(x= name,
                               xend = name,
                               y= ci99lwr,
                               yend = ci99upr),
                 size=1,col=my.cols[1]) +
  labs(y="Estimate",x="Group") +
  theme_minimal() +
  scale_colour_manual(values = my.cols, labels = names(my.cols)) +
  labs(colour = "Colour Variable")

Customizing the Legend

The scale_colour_manual function allows us to customize the legend in several ways. We can specify a vector of colors, with each color associated with a specific variable or category. We can also specify custom labels for each color.

# Create a new data frame for our plot
foo <- tibble(name = letters[1:2],
              ci90lwr = c(0.8, 0.9),
              ci90upr = c(1.2, 1.3),
              ci95lwr = c(0.7, 0.8),
              ci95upr = c(1.3, 1.4),
              ci99lwr = c(0.6, 0.7),
              ci99upr = c(1.4, 1.5))

# Create a custom legend using scale_colour_manual
foo %>%
  ggplot() +
    geom_segment(mapping = aes(x= name,
                               xend = name,
                               y= ci90lwr,
                               yend = ci90upr),
                 size=5,col=my.cols[3]) +
    geom_segment(mapping = aes(x= name,
                               xend = name,
                               y= ci95lwr,
                               yend = ci95upr),
                 size=3,col=my.cols[2]) +
    geom_segment(mapping = aes(x= name,
                               xend = name,
                               y= ci99lwr,
                               yend = ci99upr),
                 size=1,col=my.cols[1]) +
  labs(y="Estimate",x="Group") +
  theme_minimal() +
  scale_colour_manual(values = my.cols, labels = c("CI 99", "CI 95", "CI 90")) +
  labs(colour = "Colour Variable")

Conclusion

Creating a custom legend in ggplot2 is an essential skill for any data visualization enthusiast. By using the scale_colour_manual function, we can customize the color map and add labels to our legend. This allows us to associate different colors with specific variables or categories in our data. In this article, we have shown how to create a custom legend from scratch using ggplot2’s scale_colour_manual function.

Example Use Cases

Here are some example use cases for creating a custom legend in ggplot2:

Creating a color gradient map for a categorical variable
Customizing the color scheme for a scatter plot or bar chart
Adding labels to a legend for a vector of colors

Advanced Topics

There are several advanced topics related to custom legends in ggplot2. These include:

Using scale_colour_manual with multiple aesthetics
Creating a custom color map using R’s built-in color palettes
Using scale_colour_manual with custom labels and titles

Last modified on 2023-12-06