Advanced Data Visualization with ggplot2

Review

# Load packages
library(readr)
library(dplyr)

# Read the data
gm <- read_csv(file="data/gapminder.csv")

# Show the first few lines of the data
gm

## # A tibble: 1,704 x 6
##    country     continent  year lifeExp      pop gdpPercap
##    <chr>       <chr>     <dbl>   <dbl>    <dbl>     <dbl>
##  1 Afghanistan Asia       1952    28.8  8425333      779.
##  2 Afghanistan Asia       1957    30.3  9240934      821.
##  3 Afghanistan Asia       1962    32.0 10267083      853.
##  4 Afghanistan Asia       1967    34.0 11537966      836.
##  5 Afghanistan Asia       1972    36.1 13079460      740.
##  6 Afghanistan Asia       1977    38.4 14880372      786.
##  7 Afghanistan Asia       1982    39.9 12881816      978.
##  8 Afghanistan Asia       1987    40.8 13867957      852.
##  9 Afghanistan Asia       1992    41.7 16317921      649.
## 10 Afghanistan Asia       1997    41.8 22227415      635.
## # ... with 1,694 more rows

# Optionally bring up data in a viewer window.
# View(gm)

About ggplot2

ggplot2 is a widely used R package that extends R’s visualization capabilities. It takes the hassle out of things like creating legends, mapping other variables to scales like color, or faceting plots into small multiples. We’ll learn about what all these things mean shortly.

Where does the “gg” in ggplot2 come from? The ggplot2 package provides an R implementation of Leland Wilkinson’s Grammar of Graphics (1999). The Grammar of Graphics allows you to think beyond the garden variety plot types (e.g. scatterplot, barplot) and the consider the components that make up a plot or graphic, such as how data are represented on the plot (as lines, points, etc.), how variables are mapped to coordinates or plotting shape or color, what transformation or statistical summary is required, and so on.

Specifically, ggplot2 allows you to build a plot layer-by-layer by specifying:

• a geom, which specifies how the data are represented on the plot (points, lines, bars, etc.),
• aesthetics that map variables in the data to axes on the plot or to plotting size, shape, color, etc.,
• a stat, a statistical transformation or summary of the data applied prior to plotting,
• facets, which we’ve already seen above, that allow the data to be divided into chunks on the basis of other categorical or continuous variables and the same plot drawn for each chunk.

Finally, see this course’s help page for links to getting more help with ggplot2.

Plotting bivariate data: continuous Y by continuous X

The ggplot function has two required arguments: the data used for creating the plot, and an aesthetic mapping to describe how variables in said data are mapped to things we can see on the plot.

First let’s load the package:

library(ggplot2)

Now, let’s lay out the plot. If we want to plot a continuous Y variable by a continuous X variable we’re probably most interested in a scatter plot. Here, we’re telling ggplot that we want to use the gm dataset, and the aesthetic mapping will map gdpPercap onto the x-axis and lifeExp onto the y-axis. Remember that the variable names are case sensitive!

ggplot(gm, aes(x = gdpPercap, y = lifeExp))

When we do that we get a blank canvas with no data showing (you might get an error if you’re using an old version of ggplot2). That’s because all we’ve done is laid out a two-dimensional plot specifying what goes on the x and y axes, but we haven’t told it what kind of geometric object to plot. The obvious choice here is a point. Check out docs.ggplot2.org to see what kind of geoms are available.

ggplot(gm, aes(x = gdpPercap, y = lifeExp)) + geom_point()

Here, we’ve built our plot in layers. First, we create a canvas for plotting layers to come using the ggplot function, specifying which data to use (here, the gm data frame), and an aesthetic mapping of gdpPercap to the x-axis and lifeExp to the y-axis. We next add a layer to the plot, specifying a geom, or a way of visually representing the aesthetic mapping.

Now, the typical workflow for building up a ggplot2 plot is to first construct the figure and save that to a variable (for example, p), and as you’re experimenting, you can continue to re-define the p object as you develop “keeper commands”.

First, let’s construct the graphic. Notice that we don’t have to specify x= and y= if we specify the arguments in the correct order (x is first, y is second).

p <- ggplot(gm, aes(gdpPercap, lifeExp))

The p object now contains the canvas, but nothing else. Try displaying it by just running p. Let’s experiment with adding points and a different scale to the x-axis.

# Experiment with adding poings
p + geom_point()

# Experiment with a different scale
p + geom_point() + scale_x_log10()

I like the look of using a log scale for the x-axis. Let’s make that stick.

p <- p + scale_x_log10()

Now, if we re-ran p still nothing would show up because the p object just contains a blank canvas. Now, re-plot again with a layer of points:

p + geom_point()

Now notice what I’ve saved to p at this point: only the basic plot layout and the log10 mapping on the x-axis. I didn’t save any layers yet because I want to fiddle around with the points for a bit first.

Above we implied the aesthetic mappings for the x- and y- axis should be gdpPercap and lifeExp, but we can also add aesthetic mappings to the geoms themselves. For instance, what if we wanted to color the points by the value of another variable in the dataset, say, continent?

p + geom_point(aes(color=continent))

Notice the difference here. If I wanted the colors to be some static value, I wouldn’t wrap that in a call to aes(). I would just specify it outright. Same thing with other features of the points. For example, lets make all the points huge (size=8) blue (color="blue") semitransparent (alpha=(1/4)) triangles (pch=17):

p + geom_point(color="blue", pch=17, size=8, alpha=1/4)

Now, this time, let’s map the aesthetics of the point character to certain features of the data. For instance, let’s give the points different colors and character shapes according to the continent, and map the size of the point onto the life Expectancy:

p + geom_point(aes(col=continent, shape=continent, size=lifeExp))

Now, this isn’t a great plot because there are several aesthetic mappings that are redundant. Life expectancy is mapped to both the y-axis and the size of the points – the size mapping is superfluous. Similarly, continent is mapped to both the color and the point character (the shape is superfluous). Let’s get rid of that, but let’s make the points a little bigger outsize of an aesthetic mapping.

p + geom_point(aes(col=continent), size=3)

EXERCISE 1

Re-create this same plot from scratch without saving anything to a variable. That is, start from the ggplot call.

• Start with the ggplot() function.
• Use the gm data.
• Map gdpPercap to the x-axis and lifeExp to the y-axis.
• Add points to the plot
  • Make the points size 3
  • Map continent onto the aesthetics of the point
• Use a log₁₀ scale for the x-axis.

p <- ggplot(gm, aes(gdpPercap, lifeExp)) + scale_x_log10()
p + geom_point() + geom_smooth()

p + geom_point() + geom_smooth(lwd=2, se=FALSE, method="lm", col="red")

p + geom_point(aes(color = continent)) + geom_smooth()

p + aes(color = continent) + geom_point() + geom_smooth()
p + aes(color = continent) + geom_point() + geom_smooth(se=F, lwd=2)

p + geom_point() + facet_wrap(~continent)
p + geom_point() + geom_smooth() + facet_wrap(~continent, ncol=1)

p + geom_point()
ggsave(file="myplot.png")

pfinal <- p + geom_point() + geom_smooth() + facet_wrap(~continent, ncol=1)
ggsave(pfinal, file="myplot.pdf", width=5, height=15)

Plotting bivariate data: continuous Y by categorical X

With the last example we examined the relationship between a continuous Y variable against a continuous X variable. A scatter plot was the obvious kind of data visualization. But what if we wanted to visualize a continuous Y variable against a categorical X variable? We sort of saw what that looked like in the last exercise. year is a continuous variable, but in this dataset, it’s broken up into 5-year segments, so you could almost think of each year as a categorical variable. But a better example would be life expectancy against continent or country.

First, let’s set up the basic plot:

p <- ggplot(gm, aes(continent, lifeExp)) 

Then add points:

p + geom_point()

That’s not terribly useful. There’s a big overplotting problem. We can try to solve with transparency:

p + geom_point(alpha=1/4)

But that really only gets us so far. What if we spread things out by adding a little bit of horizontal noise (aka “jitter”) to the data.

p + geom_jitter()

Note that the little bit of horizontal noise that’s added to the jitter is random. If you run that command over and over again, each time it will look slightly different. The idea is to visualize the density at each vertical position, and spreading out the points horizontally allows you to do that. If there were still lots of over-plotting you might think about adding some transparency by setting the alpha= value for the jitter.

p + geom_jitter(alpha=1/2)

Probably a more common visualization is to show a box plot:

p + geom_boxplot()

But why not show the summary and the raw data?

p + geom_jitter() + geom_boxplot()

Notice how in that example we first added the jitter layer then added the boxplot layer. But the boxplot is now superimposed over the jitter layer. Let’s make the jitter layer go on top. Also, go back to just the boxplots. Notice that the outliers are represented as points. But there’s no distinction between the outlier point from the boxplot geom and all the other points from the jitter geom. Let’s change that. Notice the British spelling.

p + geom_boxplot(outlier.colour = "red") + geom_jitter(alpha=1/2)

There’s another geom that’s useful here, called a voilin plot.

p + geom_violin()

p + geom_violin() + geom_jitter(alpha=1/2)

Let’s go back to our boxplot for a moment.

p + geom_boxplot()

This plot would be a lot more effective if the continents were shown in some sort of order other than alphabetical. To do that, we’ll have to go back to our basic build of the plot again and use the reorder function in our original aesthetic mapping. Here, reorder is taking the first variable, which is some categorical variable, and ordering it by the level of the mean of the second variable, which is a continuous variable. It looks like this

p <- ggplot(gm, aes(x=reorder(continent, lifeExp), y=lifeExp))

p + geom_boxplot()

EXERCISE 3

1. Make a jittered strip plot of GDP per capita against continent.
2. Make a box plot of GDP per capita against continent.
3. Using a log₁₀ y-axis scale, overlay semitransparent jittered points on top of box plots, where outlying points are colored.
4. BONUS: Try to reorder the continents on the x-axis by GDP per capita. Why isn’t this working as expected? See ?reorder for clues.

Plotting univariate continuous data

What if we just wanted to visualize distribution of a single continuous variable? A histogram is the usual go-to visualization. Here we only have one aesthetic mapping instead of two.

p <- ggplot(gm, aes(lifeExp))

p + geom_histogram()

When we do this ggplot lets us know that we’re automatically selecting the width of the bins, and we might want to think about this a little further.

p + geom_histogram(bins=30)
p + geom_histogram(bins=10)
p + geom_histogram(bins=200)

p + geom_histogram(bins=60)

Alternative we could plot a smoothed density curve instead of a histogram:

p + geom_density()

Back to histograms. What if we wanted to color this by continent?

p + geom_histogram(aes(color=continent))

That’s not what we had in mind. That’s just the outline of the bars. We want to change the fill color of the bars.

p + geom_histogram(aes(fill=continent))

Well, that’s not exactly what we want either. If you look at the help for ?geom_histogram you’ll see that by default it stacks overlapping points. This isn’t really an effective visualization. Let’s change the position argument.

p + geom_histogram(aes(fill=continent), position="identity")

But the problem there is that the histograms are blocking each other. What if we tried transparency?

p + geom_histogram(aes(fill=continent), position="identity", alpha=1/3)

That’s somewhat helpful, and might work for two distributions, but it gets cumbersome with 5. Let’s go back and try this with density plots, first changing the color of the line:

p + geom_density(aes(color=continent))

Then by changing the color of the fill and setting the transparency to 25%:

p + geom_density(aes(fill=continent), alpha=1/4)

EXERCISE 4

1. Plot a histogram of GDP Per Capita.
2. Do the same but use a log₁₀ x-axis.
3. Still on the log₁₀ x-axis scale, try a density plot mapping continent to the fill of each density distribution, and reduce the opacity.
4. Still on the log₁₀ x-axis scale, make a histogram faceted by continent and filled by continent. Facet with a single column (see ?facet_wrap for help).
5. Save this figure to a 6x10 PDF file.

Publication-ready plots & themes

Let’s make a plot we made earlier (life expectancy versus the log of GDP per capita with points colored by continent with lowess smooth curves overlaid without the standard error ribbon):

p <- ggplot(gm, aes(gdpPercap, lifeExp)) 
p <- p + scale_x_log10()
p <- p + aes(col=continent) + geom_point() + geom_smooth(lwd=2, se=FALSE)

Give the plot a title and axis labels:

p <- p + ggtitle("Life expectancy vs GDP by Continent")
p <- p + xlab("GDP Per Capita (USD)") + ylab("Life Expectancy (years)")

By default, the “gray” theme is the usual background (I’ve changed this course website to use the black and white background for all images).

p + theme_gray()

We could also get a black and white background:

p + theme_bw()

Or go a step further and remove the gridlines:

p + theme_classic()

Finally, there’s another package that gives us lots of different themes. Install it if you don’t have it already. Install all its dependencies along with it.

install.packages("ggthemes", dependencies = TRUE)

library(ggthemes)
p <- ggplot(gm, aes(gdpPercap, lifeExp)) 
p <- p + scale_x_log10()
p <- p + aes(col=continent) + geom_point() + geom_smooth(lwd=2, se=FALSE)
p + theme_excel()
p + theme_excel() + scale_colour_excel()
p + theme_gdocs() + scale_colour_gdocs()
p + theme_stata() + scale_colour_stata()
p + theme_wsj() + scale_colour_wsj()
p + theme_economist() 
p + theme_fivethirtyeight()
p + theme_tufte()

Don’t forget - see this course’s help page for links to getting more help with ggplot2!