This is a lesson introducing you to making plots with the R package ggplot2. The ggplot2 package was originally developed by Hadley Wickham and is now developed and maintained by a huge team of data visualization experts. It’s an elegant and powerful way of visualizing your data and works great for everything from quick exploratory plots to carefully formatted publication-quality graphics.
Students should already have a beginner-level knowledge of R, including basic knowledge of functions and syntax, and awareness of how data frames in R work.
At the end of this course, you will know …
geoms to make scatterplots, boxplots, histograms, density plots, and barplots
The theory underlying ggplot2 is the “grammar of graphics.” This concept was originally introduced by Leland Wilkinson in a landmark book. It’s a formal way of mapping variables in a dataset to graphical elements of a plot. For example, you might have a dataset with age, weight, and sex of many individuals. You could make a scatterplot where the age variable in the data maps to the x axis of the dataset, the weight variable maps to the y axis, and the sex variable maps to the color of the points.
In the grammar of graphics, a plot is built in a modular way. We
start with data, map variables to visual elements called
geoms, and then optionally modify the coordinate system and
scales like axes and color gradients. We can also modify the visual
appearance of the plot in ways that don’t map back to the data, but just
make the plot look better.
If that doesn’t make sense to you, read on to see how this is implemented in ggplot2.
These images are taken from the ggplot2 cheatsheet. I recommend downloading this cheatsheet and keeping it handy – it’s a great reference!
ggplot2 uses the grammar of graphics to build up all plots from the same set of building blocks. You specify which variables in the data correspond to which visual properties (aesthetics) of the things that are being plotted (geoms).
In practice, that looks like this:
As you can see we at least need data, a mapping of variables to
visual properties (called
aes), and one or more
geom layers. Optionally, we can add coordinate system
transformations, scale transformations, facets to split the plot into
groups, and themes to change the plot appearance. We will cover all of
this (other than coordinate transformation) in this intro lesson.
In this lesson we’ll use three fun datasets from Kaggle, a data science competition site where users upload public domain datasets. Click on each link if you want to learn more about each dataset, including descriptions of each column.
We will use only the ggplot2 package in this
tutorial. Use the
read.csv() function from base R to read
in each of the three datasets from the URL where they are hosted on
library(ggplot2) WHR <- read.csv('https://github.com/qdread/data-viz-basics/raw/main/datasets/WHR_2015.csv') cereal <- read.csv('https://github.com/qdread/data-viz-basics/raw/main/datasets/cereal.csv') olympics <- read.csv('https://github.com/qdread/data-viz-basics/raw/main/datasets/olympics.csv')
You can use the
str() functions to examine each dataset.
Let’s start with a simple scatterplot. Does money buy happiness? We
will find out. The
WHR dataset has a row for each country.
We will make a scatterplot with plotting GDP per capita on the
x axis and happiness score on the y axis.
Start by calling the
ggplot() function with the
data argument saying which data frame contains the plotting
ggplot(data = WHR)
That didn’t do anything so far. We’ve only specified the dataset to get the plotting data from, without saying which columns of the dataset will be mapped to which graphical elements.
ggplot(data = WHR, aes(x = GDP.per.Capita, y = Happiness.Score))
Once we add the x and y mappings, we now can see
the two axes and coordinate system, which is already set to the range of
each variable, but no data yet. We haven’t added any
ggplot(data = WHR, aes(x = GDP.per.Capita, y = Happiness.Score)) + geom_point()
By adding a
geom_point() layer to the plotting code, we
have now made a scatterplot!
Notice we use
+to add each new piece of the plotting code.
We can modify the plot in many ways. One way is by changing the
geom. This will plot the same data but using a different
type of plot. For example we might want to connect data points with
lines instead of drawing them as separate points. For that we will
ggplot(data = WHR, aes(x = GDP.per.Capita, y = Happiness.Score)) + geom_line()
geom_line doesn’t make a lot of sense in this case
but it is great for time series data.
We can add multiple
geoms if we want. For instance, we
can plot a smoothing trendline (
geom_smooth()) overlaid on
ggplot(data = WHR, aes(x = GDP.per.Capita, y = Happiness.Score)) + geom_point() + geom_smooth()
## `geom_smooth()` using method = 'loess' and formula = 'y ~ x'
Now we have a point plot with a trendline overlaid on top. The
geoms are drawn in the order they are added to the
Notice I have put each piece on its own line. This makes the code much easier to read especially if you are making a complex plot with dozens of lines of code.
By default, the
geom_smooth() plots a locally weighted
regression with standard error as a shaded area. We can change the type
of trend to a linear trend by specifying
method = lm as an
argument, and get rid of the standard error shading by specifying
se = FALSE.
ggplot(data = WHR, aes(x = GDP.per.Capita, y = Happiness.Score)) + geom_point() + geom_smooth(method = lm, se = FALSE)
## `geom_smooth()` using formula = 'y ~ x'
If we add to or change the
aes arguments, we will modify
or change what data are used to plot. For example let’s add a
color aesthetic to the point plot to color each country’s
point by continent.
ggplot(data = WHR, aes(x = GDP.per.Capita, y = Happiness.Score, color = Continent)) + geom_point() + geom_smooth(method = lm, se = FALSE)
## `geom_smooth()` using formula = 'y ~ x'