ggplot2 Fundamentals

ggplot2 is R's most popular plotting package, built on the "grammar of graphics" — you describe a chart as data, aesthetic mappings, and geometric layers, then build it up with +.

Learn ggplot2 Fundamentals in our free R course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free R course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

By the end of this lesson you'll create scatter plots and bar charts, map columns to aesthetics with aes(), add layers like trend lines and labels, and apply a clean theme.

What You'll Learn in This Lesson

1️⃣ Your First Plot: Data + aes + geom

Every plot starts with ggplot(data, aes(...)) to set the data and mappings, then adds at least one geom_ layer to draw it. geom_point() makes a scatter plot.

2️⃣ Adding Layers

Plots grow by adding layers with + . Style a geom directly (colour, size), overlay a trend line with geom_smooth() , label with labs() , and clean up with a theme.

3️⃣ Bar Charts: geom_bar vs geom_col

Use geom_bar() to count how many rows fall in each category, and geom_col() when you already have the value to plot per category.

Your turn. Fill in the # TODO blank, run it, and check the Plots pane.

Write it from the outline using the built-in mtcars data, run it in RStudio, and check the plot. Mapping colour to a factor is how you reveal groups in a scatter.

📋 Quick Reference — ggplot2

Practice quiz

Which three parts make up every ggplot?

Rows, columns, cells
Title, axes, legend
Mean, median, mode
Data, aesthetic mappings, and a geom layer

Answer: Data, aesthetic mappings, and a geom layer. A ggplot is built from data, aes() mappings, and one or more geom layers.

What does aes() do?

Maps data columns to visual properties like x, y, colour
Saves the plot
Loads the package
Sets the file size

Answer: Maps data columns to visual properties like x, y, colour. aes() connects data columns to aesthetics such as x, y, and colour.

Which geom draws a scatter plot?

geom_bar()
geom_point()
geom_line()
geom_col()

Answer: geom_point(). geom_point() plots individual points for a scatter plot.

How do you add layers to a ggplot?

With the | operator
With commas
With the + operator
With %>%

Answer: With the + operator. ggplot2 stacks layers with +, each on the end of the previous line.

What is the difference between geom_bar() and geom_col()?

They are identical
geom_bar is for lines, geom_col for points
geom_bar counts rows per category; geom_col uses a supplied y value
geom_col counts rows; geom_bar uses a y value

Answer: geom_bar counts rows per category; geom_col uses a supplied y value. geom_bar() counts frequencies; geom_col() draws bars from a y value you provide.

Which function loads ggplot2 for the session?

library(ggplot2)
import ggplot2
load(ggplot2)
use(ggplot2)

Answer: library(ggplot2). library(ggplot2) attaches the package; install once with install.packages.

To set ONE fixed colour for all points, where does colour go?

Inside aes()
Outside aes(), in the geom, e.g. geom_point(color = "blue")
In labs()
In a theme() call

Answer: Outside aes(), in the geom, e.g. geom_point(color = "blue"). A constant style goes outside aes(); only data-driven colour goes inside aes().

Which layer adds a linear best-fit trend line?

geom_trend()
geom_fit()
geom_smooth(method = "lm")
geom_line(lm = TRUE)

Answer: geom_smooth(method = "lm"). geom_smooth(method = "lm") overlays a straight-line fit.

A common beginner mistake with + is:

Putting + at the START of the next line instead of the end of the previous one
Using + only once
Using + with geoms only
Never needing +

Answer: Putting + at the START of the next line instead of the end of the previous one. The + must END the previous line, or R thinks the plot is finished.

Which function sets the plot title and axis labels?

title()
theme()
aes()
labs()

Answer: labs(). labs(title = ..., x = ..., y = ...) sets the plot's text labels.