Align Plot with Different Axes Vertically Using Cowplot in R

When working with data visualization in R, you often encounter scenarios where you need to align multiple plots vertically, even if they have different axis scales or dimensions. The cowplot package is particularly useful for combining and aligning such plots seamlessly. This article provides a detailed guide on how to align plots with different axes vertically using the cowplot package.

Introduction to Cowplot

The cowplot the package extends the functionality of ggplot2, offering more flexibility in arranging multiple plots. It’s designed for easy integration with ggplot2 and provides simple functions to arrange multiple plots in various configurations. To begin using cowplot, you need to install and load it:

# Install and load the cowplot package
install.packages("cowplot")
library(cowplot)

Now we will discuss step by step implementation of Align Plot with Different Axes Vertically Using Cowplot in R Programming Language.

Step 1: Creating Sample Data and Plots

For demonstration, let's create two different plots using the ggplot2 package:

• A scatter plot
• A bar plot with different y-axis scaling.

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

# Create sample data
data1 <- data.frame(x = 1:10, y = c(3, 5, 2, 8, 7, 9, 4, 6, 10, 12))
data2 <- data.frame(category = c("A", "B", "C", "D"), value = c(15, 25, 10, 30))

# Create a scatter plot
plot1 <- ggplot(data1, aes(x = x, y = y)) +
  geom_point(color = "blue", size = 3) +
  labs(title = "Scatter Plot", x = "X Axis", y = "Y Axis (Scatter)") +
  theme_minimal()

# Create a bar plot
plot2 <- ggplot(data2, aes(x = category, y = value)) +
  geom_bar(stat = "identity", fill = "orange") +
  labs(title = "Bar Plot", x = "Category", y = "Y Axis (Bar)") +
  theme_minimal()

At this point, we have two distinct plots (plot1 and plot2), each with different y-axis scales.

Step 2: Aligning Plots Vertically Using Cowplot

The main function in cowplot used to align plots vertically is plot_grid(). This function enables you to combine multiple plots into a single layout and align their axes.

# Aligning the two plots vertically using cowplot
combined_plot <- plot_grid(
  plot1,       # First plot
  plot2,       # Second plot
  ncol = 1,    # Arrange in one column (i.e., vertically)
  align = "v", # Align vertically
  axis = "lr"  # Align left ('l') and right ('r') axes
)

# Display the combined plot
print(combined_plot)

Output:

• ncol = 1 ensures the plots are arranged vertically.
• align = "v" specifies vertical alignment.
• axis = "lr" aligns both left and right y-axes.

Step 3: Adjusting Plot Heights

Sometimes, the default heights of the aligned plots may not be ideal. You can adjust the relative heights of the plots using the rel_heights parameter.

# Adjusting the relative heights of the plots
combined_plot_adjusted <- plot_grid(
  plot1,
  plot2,
  ncol = 1,
  align = "v",
  axis = "lr",
  rel_heights = c(1, 1.5) # Adjust heights: 1 for plot1, 1.5 for plot2
)

# Display the adjusted combined plot
print(combined_plot_adjusted)

Output:

In this example, rel_heights = c(1, 1.5) makes plot2 1.5 times taller than plot1.

Step 4: Adding Titles and Annotations

The cowplot package allows you to add titles or annotations to the combined plot using draw_plot_label() or ggdraw().

# Adding a title to the combined plot
combined_with_title <- ggdraw() +
  draw_plot(combined_plot) +
  draw_label("Combined Plot with Different Axes", x = 0.6 , y = 0.95, hjust = 0.5, size = 12)

# Display the combined plot with a title
print(combined_with_title)

Output:

Conclusion

Aligning plots with different axes vertically in R using the cowplot package is a straightforward process that enhances the presentation of multiple plots in a single layout. This is especially useful for comparative analysis where you need to display plots with distinct scales side by side. By leveraging plot_grid() along with other cowplot functions, you can create visually appealing and professional statistical reports.