How to Use facet_grid in ggplot2 for Plot Faceting

Use facet_grid(rows ~ cols) in ggplot2 to split your plot into a grid of panels based on the values of one or two categorical variables. The rows and cols specify which variables define the grid's rows and columns, respectively. This helps compare subsets of data side-by-side in a clear layout.

Syntax

The basic syntax of facet_grid is facet_grid(rows ~ cols), where:

• rows: variable(s) to create rows of panels
• cols: variable(s) to create columns of panels

You can use a single variable or . to indicate no faceting on that dimension.

r

facet_grid(rows ~ cols)

Example

This example shows how to use facet_grid to create a grid of scatter plots split by two categorical variables from the built-in mtcars dataset.

r

library(ggplot2)

# Basic scatter plot with facet_grid
p <- ggplot(mtcars, aes(x = wt, y = mpg)) +
  geom_point() +
  facet_grid(cyl ~ gear) +
  labs(title = "MPG vs Weight faceted by Cylinders and Gears")

print(p)

Output

A scatter plot with multiple panels arranged in a grid where rows represent different cylinder counts (cyl) and columns represent different gear counts (gear). Each panel shows points of weight vs mpg for that subgroup.

Common Pitfalls

• Using facet_grid with continuous variables will not work well; it expects categorical variables.
• Using . in place of rows or columns means no faceting on that axis.
• Confusing facet_grid with facet_wrap: facet_grid creates a strict grid, while facet_wrap wraps panels in one direction.

r

library(ggplot2)

# Wrong: using continuous variable in facet_grid
# This will produce an error or unexpected results
# ggplot(mtcars, aes(wt, mpg)) + geom_point() + facet_grid(wt ~ cyl)

# Right: use categorical variables only
p <- ggplot(mtcars, aes(wt, mpg)) + geom_point() + facet_grid(cyl ~ gear)
print(p)

Output

The correct plot shows faceting by cylinders and gears; the wrong code is commented out because it causes errors or warnings.

Quick Reference

Usage	Description
facet_grid(rows ~ cols)	Create a grid of panels by rows and columns variables
facet_grid(. ~ var)	Facet only by columns using variable 'var'
facet_grid(var ~ .)	Facet only by rows using variable 'var'
facet_grid(rows + rows2 ~ cols)	Facet by multiple variables in rows or columns
Use categorical variables	Ensure faceting variables are factors or discrete
Use facet_wrap for one-dimensional faceting	When you want panels wrapped in one direction

Key Takeaways

Use facet_grid(rows ~ cols) to split plots into a grid by categorical variables.

Use a dot (.) to skip faceting on rows or columns.

Only use categorical variables for faceting with facet_grid.

facet_grid creates a strict grid layout, different from facet_wrap.

Check your variables are factors or discrete for proper faceting.