Adding Graphics to ggplot Using a Loop

Problem and Its Causes

When working with the ggplot2 library in R, there arises a situation where creating plots inside a loop leads to incorrect image display. If the plots are created separately, they are displayed correctly, but when attempting to create them through a loop, all plots become identical—images that correspond to the last plot.

Description of the Problem

Here is an example code that demonstrates the problem:

graphs <- list()
categories <- data.frame(category = c("cat1", "cat2", "cat3", "cat4"))
data <- data.frame(...)  # Assume this data frame is populated with appropriate values

for (i in 1:length(categories$category)) {
    graphs[[i]] <- data %>%
        ggplot(aes(x = x_var, y = y_var)) +
        geom_col() +
        geom_image(
            data = tibble(image_path = paste0(categories$category[i], ".png")),
            aes(image = image_path)
        )
}

ggarrange(plotlist = graphs, ncol = 1, spacing = "0")

The issue lies in how geom_image may not properly handle data within a loop. It's possible that the images are not updated for each plot individually.

Solution to the Problem

To solve this problem, you can use the function ggplot2::ggsave() to save each plot separately and then combine them using ggarrange. This ensures that each image is correctly processed before it is added to other plots.

Steps to Solve:

1. Saving Plots Separately: Use ggsave() to save each plot to a separate file.
2. Combining Plots: Then load these files and combine them using ggarrange.

Here is the corrected code:

graphs <- list()
categories <- data.frame(category = c("cat1", "cat2", "cat3", "cat4"))
data <- data.frame(...)  # Assume this data frame is populated with appropriate values

for (i in 1:length(categories$category)) {
    plot <- data %>%
        ggplot(aes(x = x_var, y = y_var)) +
        geom_col() +
        geom_image(
            data = tibble(image_path = paste0(categories$category[i], ".png")),
            aes(image = image_path)
        ) +
        ggsave(paste0("temp_plot_", i, ".png"), width = 5, height = 5)
    
    graphs[[i]] <- readPNG(paste0("temp_plot_", i, ".png"))
}

library(ggpubr)
ggarrange(plotlist = graphs, ncol = 1, spacing = "0")

Practical Tips

• Check Image Paths: Ensure that the paths to the images are correct and accessible.
• Use Temporary Files: Saving plots to temporary files helps avoid issues with updating data.
• Pay Attention to Parameters: Parameters such as width, height, and spacing can affect the appearance of the result.

Conclusion

By following these steps, you can successfully combine plots with images created in a loop using ggplot2 and ggpubr. This approach ensures the correct display of all images without errors.