Exercise: Introduction to `ggplot2`

Author

Affiliation

Valerio Licursi

IBPM-CNR

Published

June 26, 2024

Objective:

Learn how to create and customize basic plots using the ggplot2 package.

Prerequisites:

Basic knowledge of R and data frames
ggplot2 package installed (install.packages("ggplot2"))

Exercise Steps:

1. Install and Load `ggplot2`

First, ensure you have the ggplot2 package installed and loaded.

# install.packages("ggplot2") # don't execute it if you have already installed the package
library(ggplot2)

2. Load the `mtcars` Dataset

For this exercise, we will use the built-in mtcars dataset.

data(mtcars)
head(mtcars)

                   mpg cyl disp  hp drat    wt  qsec vs am gear carb
Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

3. Create a Basic Scatter Plot

Use ggplot to create a scatter plot of mpg (miles per gallon) vs wt (weight of the car).

ggplot(data = mtcars, aes(x = wt, y = mpg)) +
  geom_point()

4. Add Aesthetics

Enhance your scatter plot by adding color to differentiate between the number of cylinders (cyl).

ggplot(data = mtcars, aes(x = wt, y = mpg, color = as.factor(cyl))) +
  geom_point()

5. Add Titles and Labels

Add a title, and labels for the x and y axes.

ggplot(data = mtcars, aes(x = wt, y = mpg, color = as.factor(cyl))) +
  geom_point() +
  ggtitle("MPG vs Weight of Cars") +
  xlab("Weight (1000 lbs)") +
  ylab("Miles per Gallon") +
  labs(color = "Cylinders")

6. Create a Boxplot

Create a boxplot to visualize the distribution of mpg for each cyl category.

ggplot(data = mtcars, aes(x = as.factor(cyl), y = mpg)) +
  geom_boxplot() +
  xlab("Cylinders") +
  ylab("Miles per Gallon") +
  ggtitle("Boxplot of MPG by Cylinders")

7. Create a Histogram

Create a histogram to visualize the distribution of mpg.

ggplot(data = mtcars, aes(x = mpg)) +
  geom_histogram(binwidth = 2, fill = "blue", color = "black") +
  xlab("Miles per Gallon") +
  ggtitle("Histogram of MPG")

8. Create a Faceted Plot

Use faceting to create multiple plots based on the number of cylinders (cyl).

ggplot(data = mtcars, aes(x = wt, y = mpg)) +
  geom_point() +
  facet_wrap(~ cyl) +
  xlab("Weight (1000 lbs)") +
  ylab("Miles per Gallon") +
  ggtitle("MPG vs Weight Faceted by Number of Cylinders")

Additional Customizations:

Change Themes: Try different themes such as theme_minimal(), theme_classic(), etc.
Modify Point Shapes and Sizes: Use the shape and size aesthetics in geom_point().
Save the Plot: Use ggsave() to save your plot to a file.