Understanding Facet Grids in ggplot2

Facet grids are a powerful feature in the ggplot2 package for creating complex and interactive visualizations. In this article, we will explore how to customize the default labels in facet grid output.

Introduction to Facets and Labels

In faceted plots, each facet represents a different group or category of data. The facet_grid() function allows us to create multiple facets with different variables on the x-axis and y-axis. By default, ggplot2 uses a simple label system for each facet, which can be customized using the labeller argument.

One common issue when working with facet grids is how to handle missing values (NA) in the labels of individual facets. The default behavior is to display NA as “(N/A)” or similar labels, which may not always be desirable.

Why Can’t We Change Default Labels?

The reason we can’t change the default label for NA directly lies in the underlying structure of the faceted plot. When creating a facet grid, ggplot2 uses a separate labs() object to define the labels for each axis and facet. However, when it comes to displaying labels for missing values (NA), there is no explicit mechanism provided by the function.

This limitation means that we need to find alternative solutions to handle NA labels in our plots.

Solution: Using Custom Labels

Fortunately, we can overcome this limitation by using a custom labeller object within the facet_grid() function. A labeller object allows us to map values (in this case, NA) to new labels.

Here’s how you can modify your existing code:

library(ggplot2)

set.seed(123)
df_plot <- data.frame(
  Platform_joined = sample(c("In Person", "Online"), size = 50, replace = TRUE),
  sum_mastery = rnorm(50),
  gender = sample(c("0", "1"), size = 50, replace = TRUE),
  native_speaker = sample(c("0","1"), 
                          size = 50, replace = TRUE)
)

# Create custom labels for NA
gender.labs <- c("Female","Male", "(all)")
names(gender.labs) <- c("0","1",(all))
native_speaker.labs <-c("Non Native-speaker","Native-speaker","(all)")
names(native_speaker.labs) <- c("0","1",(all))

# Create a custom labeller for facet grid
custom_labeller <- function(x) {
  ifelse(x == "NA", "(N/A)", x)
}

# Use the custom labeller to create the facet grid
ggplot(df_plot, aes(x = Platform_joined, y = sum_mastery)) +
  geom_boxplot() +
  geom_point() +
  labs(x = "Teaching Motality", y = "Total Mastery Gained") +
  theme(axis.title.x = element_blank()) +
  facet_grid(gender~native_speaker, margins = TRUE, 
             labeller = labeller(gender = custom_labeller,
                                 native_speaker = custom_labeller))

By defining a custom labeller function that checks for NA values and returns the new desired label, we can customize the labels in our facet grid.

Note: When using custom labelling for multiple faceted plots, make sure to define the same function for each variable used in your facet_grid() function.

Last modified on 2024-09-20