Making your first plot

Andrew Irwin, a.irwin@dal.ca

2024-01-16

Use the ggplot2 library

Do this once (if you haven’t done it already):

install.packages("tidyverse")

Add this line to every R markdown document:

library(tidyverse)

Get some data

glimpse(mtcars)

Rows: 32
Columns: 11
$ mpg  <dbl> 21.0, 21.0, 22.8, 21.4, 18.7, 18.1, 14.3, 24.4, 22.8, 19.2, 17.8,…
$ cyl  <dbl> 6, 6, 4, 6, 8, 6, 8, 4, 4, 6, 6, 8, 8, 8, 8, 8, 8, 4, 4, 4, 4, 8,…
$ disp <dbl> 160.0, 160.0, 108.0, 258.0, 360.0, 225.0, 360.0, 146.7, 140.8, 16…
$ hp   <dbl> 110, 110, 93, 110, 175, 105, 245, 62, 95, 123, 123, 180, 180, 180…
$ drat <dbl> 3.90, 3.90, 3.85, 3.08, 3.15, 2.76, 3.21, 3.69, 3.92, 3.92, 3.92,…
$ wt   <dbl> 2.620, 2.875, 2.320, 3.215, 3.440, 3.460, 3.570, 3.190, 3.150, 3.…
$ qsec <dbl> 16.46, 17.02, 18.61, 19.44, 17.02, 20.22, 15.84, 20.00, 22.90, 18…
$ vs   <dbl> 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 0,…
$ am   <dbl> 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 0,…
$ gear <dbl> 4, 4, 4, 3, 3, 3, 3, 4, 4, 4, 4, 3, 3, 3, 3, 3, 3, 4, 4, 4, 3, 3,…
$ carb <dbl> 4, 4, 1, 1, 2, 1, 4, 2, 2, 4, 4, 3, 3, 3, 4, 4, 4, 1, 2, 1, 1, 2,…

Also try mtcars or str(mtcars) or View(mtcars).

First plot

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

• The pipe symbol (|>) is function composition.
  • f(g(x)) can be written x |> g |> f.
• aes is a function to define aesthetic associations between features of your plot and variables in the dataset.
• Parts of a ggplot are added togther with +
• The kind of plot is called its geometry.
  • geom_point makes a scatterplot.

What if you forget … ?

mtcars |>
  ggplot(aes(x = wt,
             y = mpg))

Try “forgetting” other parts of the code to see what goes wrong.

Add some colour

mtcars |>
  ggplot(aes(x = wt,
             y = mpg,
             color = factor(cyl))) + 
  geom_point()

cyl is a number, so I must turn it into a categorical variable (factor) to get a discrete colour scale.

Try using cyl instead of factor(cyl).

Make the text larger

mtcars |>
  ggplot(aes(x = wt,
             y = mpg,
             color = factor(cyl))) + 
  geom_point() +
  theme(text = element_text(size = 28))

Make the text larger

Make the symbols larger

mtcars |>
  ggplot(aes(x = wt,
             y = mpg,
             color = factor(cyl))) + 
  geom_point(size=3) + 
  theme(text = element_text(size = 28)) 

I’m setting the size of all the symbols, not connecting a variable to the size aesthetic. So don’t use aes.

geom_point (and any other geom) inherits the aesthetics from ggplot.

Make the symbols larger

Customize the labels

mtcars |>
  ggplot(aes(x = wt,
             y = mpg,
             color = factor(cyl))) + 
  geom_point(size=3) +
  theme(text = element_text(size = 28)) +
  labs(x = "Car mass (x 1000 pounds)", 
       y = "Fuel consumption (mpg)", 
       color = "Number of\nCylinders", 
       title = "Car road test results", 
       caption = "from Motor Trend magazine") 

Customize the labels

Summary

• Start with data

• Pipe (|>) into ggplot

• Define the aesthetics: aes(x = ..., y = ..., color = ..., shape = ...)

• Define the geometry: geom_point shown here, but there are lots more

• Customize text

Suggested reading

• Course notes: Making your first plot

• Healy. Section 2.6. Make your first figure

• R4DS. Chapter 3: Data visualization

• Lots more detail: The ggplot2 book

Task and Assignment

• Try these plotting commands on your own

• Assignment 1: Your first plotting exercises

Datasets to experiment with

• mtcars, iris and many other well-known data in datasets package
• penguins in palmerpenguins package
• gapminder in gapminder package (see Gapminder website too)
• diamonds in ggplot2 package

Use glimpse(mtcars) (or other dataset) and View(mtcars) to look at the data.

Use ?mtcars to get documentation for the dataset, or use the search in the “Help” panel.

glimpse is in the dplyr package. Load it into your R session using library(tidyverse).