QQ Plot in R with ggplot2 and base R

Sample data

A QQ plot (quantile-quantile plot) compares the quantiles of your data against the theoretical quantiles of a distribution — usually normal. Points that fall along the diagonal line indicate the data follows that distribution.

set.seed(1)
df <- data.frame(x = rnorm(200))

Base R: `qqnorm()` + `qqline()`

The quickest approach is base R. qqnorm() draws the points and qqline() adds the reference line.

set.seed(1)
x <- rnorm(200)

qqnorm(x)
qqline(x, col = "red")

QQ plot in base R with qqnorm and qqline

`geom_qq()` + `geom_qq_line()`

QQ plot in ggplot2 with geom_qq and geom_qq_line

In ggplot2, use aes(sample = ...) — not x or y. geom_qq() plots the points and geom_qq_line() adds the diagonal reference line.

# install.packages("ggplot2")
library(ggplot2)

ggplot(df, aes(sample = x)) +
  geom_qq() +
  geom_qq_line()

Color and line style

Customize the points with color and size in geom_qq(), and the reference line with color and linewidth in geom_qq_line().

# install.packages("ggplot2")
library(ggplot2)

ggplot(df, aes(sample = x)) +
  geom_qq(color = "steelblue", size = 1.5) +
  geom_qq_line(color = "red", linewidth = 0.8)

QQ plot in ggplot2 with custom colors for points and reference line

Multiple groups

QQ plot with multiple groups colored in ggplot2

Add color = group inside aes() to draw one QQ plot per group. Here normal data hugs the line while exponential data curves away.

# install.packages("ggplot2")
library(ggplot2)

ggplot(df_groups, aes(sample = x, color = group)) +
  geom_qq() +
  geom_qq_line()

Use facet_wrap() to give each group its own panel. This makes it easier to judge normality per group independently.

# install.packages("ggplot2")
library(ggplot2)

ggplot(df_groups, aes(sample = x)) +
  geom_qq() +
  geom_qq_line(color = "red") +
  facet_wrap(~group)

QQ plot with facet_wrap by group in ggplot2

Non-normal data

QQ plot of right-skewed exponential data showing deviation from normality in ggplot2

When data is not normal the points curve away from the reference line. Right-skewed data (like exponential) curves upward in the upper tail.

# install.packages("ggplot2")
library(ggplot2)

set.seed(1)
df_exp <- data.frame(x = rexp(200))

ggplot(df_exp, aes(sample = x)) +
  geom_qq() +
  geom_qq_line(color = "red")