Understanding Back-to-Back Bar Plots in R with Independent Axes

When it comes to visualizing data, creating effective plots is crucial for communication and interpretation. One common type of plot used to display categorical data is the bar plot. However, sometimes we need to create a back-to-back bar plot where each side is on an independent axis. In this article, we’ll explore how to achieve this in R using ggplot2.

Background: Creating Bar Plots with ggplot2

Before we dive into creating back-to-back bar plots, let’s quickly review the basics of creating bar plots using ggplot2. The ggplot() function is used to create a new plot object from the data frame passed as an argument. We can then add various layers to customize our plot.

library(ggplot2)
df <- structure(list(Description = c("a", "b", "c", "d", "e", "f",
    "g", "h", "a", "b", "c", "d", "e", "f", "g", "h"), test = c("size",
    "size", "size", "size", "size", "size", "size", "size", "p",
    "p", "p", "p", "p", "p", "p", "p"), value = c(0.1, 0.1, 0.125,
    0.1, 0.075, 0.1, 0.075, 0.125, 0.000230705311441713, 0.000314488619269942,
    0.00106639822095382, 0.00108290238851994, 0.00114723539549198,
    0.00160204850890075, 0.0019276388745184, 0.00320371567547557)), .Names =
c("Description", "test", "value"), row.names = c(NA, -16L), class =
"data.frame")

ggplot(df, aes(x=Description, y=value, fill=test)) + geom_col() +
    coord_flip()

The Problem with Back-to-Back Bar Plots

The question asks how to create a back-to-back bar plot where each side is on an independent axis. However, simply taking the negative of one set doesn’t solve the issue of having bars meet at zero in the middle.

df$value[df$test == 'p'] <- -df$value[df$test == 'p']
ggplot(df, aes(x=Description, y=value, fill=test)) + geom_col() +
    coord_flip()

Solution: Using Facets

To create a back-to-back bar plot with independent axes, we can use facets. Facets allow us to split our plot into multiple panels that share the same axes.

ggplot(df, aes(x=Description, y=value, fill=test)) + 
    facet_wrap(~test, scales = "free_x") + 
    geom_col() + 
    coord_flip() +
    scale_y_continuous(expand = c(0, 0)) +
    theme(panel.spacing.x = unit(0, "mm"))

Tweaking Facets

While using facets is an effective way to create a back-to-back bar plot with independent axes, it might lead to issues with axis labels. In this case, we need to tweak our code to remove any spacing between the facets.

ggplot(df, aes(x=Description, y=value, fill=test)) + 
    facet_wrap(~test, scales = "free_x") + 
    geom_col() + 
    coord_flip() +
    scale_y_continuous(expand = c(0, 0), labels = function(x) signif(abs(x),
        3)) +
    theme(panel.spacing.x = unit(0, "mm"))

Conclusion

Creating a back-to-back bar plot with independent axes in R using ggplot2 is achievable by leveraging facets. By tweaking our code to remove spacing between the facets and adjusting axis labels, we can create an effective visual representation of our data.

Further Reading

• ggplot2 Documentation
• Facets in ggplot2
• Scaling Axes in ggplot2

Last modified on 2025-04-30