How to Add a Complete Background Image to a ggplot in R with Custom Scaling and Positioning for SVG Export.

Introduction to ggplot2 and Background Images in R

Overview of ggplot2 and its capabilities

ggplot2 is a popular data visualization library for R, developed by Hadley Wickham. It provides an elegant and expressive syntax for creating high-quality graphics, allowing users to create complex plots with ease. One of the key features of ggplot2 is its ability to customize the appearance of plots, including adding background images.

Background Images in ggplot2

To add a background image to a plot using ggplot2, we can use the draw_image() function from the cowplot package. This function allows us to embed an image into a plot, making it a useful tool for creating visually appealing graphics.

The Problem and Our Goal

The original poster of the Stack Overflow question wants to export a ggplot with a complete background image in R. They have created a scatterplot using the mtcars dataset and want to add a background image from an open-source repository. However, their code is producing cropped images when saving the plot as an SVG.

Prerequisites: Installing Required Packages

Before we begin, make sure you have the required packages installed in your R environment:

# Install necessary packages
install.packages("ggplot2")
install.packages("cowplot")

Creating a ggplot with a Background Image

To create a ggplot with a background image, follow these steps:

Step 1: Load Required Libraries and Data

First, load the required libraries and data. We will use the ggplot2 library for plotting and the cowplot library for adding background images.

# Load necessary libraries
library(ggplot2)
library(cowplot)

# Load the mtcars dataset
data("mtcars")

Step 2: Create a Scatterplot

Next, create a scatterplot using the ggplot() function. We will use the wt column on the x-axis and the mpg column on the y-axis.

# Create a scatterplot
gg <- ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point(size = 2, shape = 23) +
  theme_minimal()

Step 3: Add a Background Image

Now, add a background image using the draw_image() function. We will use an open-source image from the Wikimedia Commons repository.

# Load the image file
library(magick)
image_file <- "images/coche.jpg"

# Add the background image
gg_draw() +
  draw_image(image_file,
             scale = 1) +
  draw_plot(gg)

Scaling and Exporting the Background Image

To understand why the original poster’s code is producing cropped images when saving the plot as an SVG, we need to examine how scaling affects the background image.

Scale of the Background Image

When scaling the background image, the scale argument controls how much the image will be stretched or compressed in both width and height directions. If you double the scale, the image will appear twice as large.

# Increase the scale to see the effect
gg_overlap <- gg_draw() +
  draw_image(image_file,
             scale = 2) +
  draw_plot(gg)

# Save the plot with the original scaling
ggsave(file = "test_original.svg", plot = gg)

Exporting a Plot and Background Image as an SVG

To export the plot and background image as an SVG, we can use the ggsave() function. However, by default, this function will only save the plot area without the background image.

# Save the plot with both the plot area and the background image
ggsave(file = "test.svg", plot = gg_overlap)

However, as the original poster has discovered, even when saving with the scale argument set to 1, the output can still be cropped. To achieve the desired result of preserving the entire background image in the exported SVG, we need to use a different approach.

Alternative Approach: Using `position = 'fill'`

One solution is to adjust the position argument of the draw_image() function to ensure that the background image covers the entire plot area.

# Adjust the position argument
gg_overlap <- gg_draw() +
  draw_image(image_file,
             scale = 1) +
  position("fill") +
  draw_plot(gg)

# Save the plot with both the plot area and the background image
ggsave(file = "test.svg", plot = gg_overlap)

By setting position = 'fill', we instruct R to place the background image so that it fills the entire available space.

Conclusion

In this article, we explored how to add a complete background image to a ggplot using the cowplot package and customized scaling. We also examined why some export methods can produce cropped images despite setting correct scaling values. Finally, we demonstrated an alternative approach using position = 'fill' to preserve the entire background image in exported SVG files.