Order Bars in ggplot2 bar graph

When creating bar graphs in ggplot2, it’s often crucial to control the order of bars on the x-axis based on a specific variable. This guide explores the theory behind ordering bars in ggplot2 bar graphs and provides various examples to illustrate different scenarios in the R Programming Language.

In ggplot2, the reorder() function is commonly used to reorder bars on the x-axis based on a specified variable. The reorder() function takes two arguments: the variable you want to reorder and the variable you want to use for ordering. It returns a factor with the levels reordered based on the values of the second argument.

R

# Sample data
data <- data.frame(
  category = c("A", "B", "C", "D", "E"),
  value = c(15, 8, 5, 12, 20)
)

Basic Bar Plot

In this example, we’ll create a basic bar plot without any specific ordering:

R

# Basic bar plot
basic_plot <- ggplot(data, aes(x = category, y = value)) +
  geom_bar(stat = "identity", fill = "skyblue") +
  labs(title = "Basic Bar Plot")

# Display the basic plot
print(basic_plot)

Output:

In this basic bar plot, the categories are plotted in the order they appear in the dataset.

Ordered Bar Plot

Now, let’s reorder the bars based on the value variable:

R

# Ordered bar plot
ordered_plot <- ggplot(data, aes(x = reorder(category, -value), y = value)) +
  geom_bar(stat = "identity", fill = "skyblue") +
  labs(title = "Ordered Bar Plot")

# Display the ordered plot
print(ordered_plot)

Output:

Here, the reorder(category, -value) function is used to reorder the bars on the x-axis based on the value variable in descending order. The resulting plot will have bars ordered from highest to lowest values.

Custom Ordering

You can also specify a custom order for the bars. In this example, let’s order the bars alphabetically:

R

# Custom ordered bar plot in ascending order based on values
custom_order_plot_ascending_values <- ggplot(data, aes(x = reorder(category, value,
                                                   FUN = function(x) sum(x)),
                                                                   y = value)) +
  geom_bar(stat = "identity", fill = "skyblue") +
  labs(title = "Custom Ordered Bar Plot (Ascending by Values)")

# Display the custom ordered plot in ascending order based on values
print(custom_order_plot_ascending_values)

Output:

The reorder() function is adjusted to include the value variable, and a custom function is provided to reorder the categories in ascending order based on their values. The FUN argument now calculates the sum of the values for each category, ensuring ascending order by the total value.

Conclusion

Ordering bars in ggplot2 bar graphs is a valuable technique for enhancing the visual representation of your data. Whether you’re emphasizing certain categories or presenting data in a specific order, the reorder() function provides flexibility in achieving your desired bar order. Adapt these examples to suit your specific dataset and visualization goals.