Data Visualization in R

Goals of the lecture

• Data visualization and exploratory data analysis (EDA).
• Basic principles of data visualization.
• ggplot: theory and practice.
• Other plotting add-ons (ggridges, ggcorrplot).

What is data visualization?

Data visualization is the process (and result) of representing data graphically.

We’ll be focusing on common visualization techniques, such as:

• Histograms.
• Scatterplots.
• Barplots.
• Boxplots.

Note

We’ll also discuss why visualization is so crucial.

Why visualization?

Data visualization serves (at least) a few different purposes:

• Exploratory data analysis (EDA): discovering relationships in your data, generating hypotheses, confirming intuitions.
  
• Communicating insights: given some finding, conveying that clearly and accurately.
• Impacting the world: a good (or bad) visualization can change attitudes!

EDA: Checking your assumptions

### Loading the tidyverse
library(tidyverse)
### Plot anscombe's quartet
anscombe %>%
  pivot_longer(
    cols = everything(),
    names_to = c(".value", "dataset"),
    names_pattern = "(x|y)(.*)"
  ) %>%
  ggplot(aes(x = x, y = y)) +
  geom_point() +
  geom_smooth(method = "lm", se = FALSE) +
  facet_wrap(~dataset)

DataViz: impacting the world (1)

Florence Nightingale (1820-1910) was a social reformer, statistician, and founder of modern nursing.

DataViz: impacting the world (2)

John Snow (1813-1858) was a physician whose visualization of cholera outbreaks helped identify the source and spreading mechanism (water supply).

What makes a good data visualization?

Edward Tufte argues:

Graphical excellence is the well-designed presentation of interesting data—a matter of substance, of statistics, and of design … [It] consists of complex ideas communicated with clarity, precision, and efficiency. … [It] is that which gives to the viewer the greatest number of ideas in the shortest time with the least ink in the smallest space … [It] is nearly always multivariate … And graphical excellence requires telling the truth about the data. (Tufte, 1983, p. 51).

Some principles:

• Use your ink wisely.
• Be true to the data.
• Consider the visual logic of the figure.
• Order matters.
• Keep scales consistent.

Principle 1: Use your ink wisely

• Every element in your visualization should serve a purpose.
• Remove “chart junk”: unnecessary gridlines, borders, 3D effects, decorations.
• Maximize your data-ink ratio: the proportion of ink used to display actual data.

Tufte’s principle

“Above all else show the data.” - Edward Tufte

Principle 2: Be true to the data

• Don’t manipulate scales to exaggerate or hide effects.
• Include zero baseline for bar charts (unless there’s good reason not to).
• Avoid cherry-picking data or timeframes.
• Represent uncertainty when appropriate (e.g., error bars, confidence intervals).

Principle 3: Consider the visual logic

• Position is probably easiest to judge accurately.
• Angle and area are harder (e.g., pie charts).
• Color hue can work for categorical data.
  • Use distinctive and meaningful colors!
• Stacked bar plots often hard to interpret!

Tip

This will be relevant when thinking about the layers in ggplot.

Principle 4: Order matters

• For categorical data: order by frequency or a meaningful sequence.
• For ordinal data: maintain the natural order (e.g., Strongly Disagree → Strongly Agree).
• For time series: always order chronologically.

Principle 5: Keep scales consistent

• Use the same axis ranges for meaningful comparison.
• In faceted plots, decide: fixed scales (scales = “fixed”) or free scales (scales = “free”)?
  • Free scales can be misleading but useful when ranges differ greatly.

Rule of thumb

Use consistent scales when inviting direct comparison; use free scales when showing patterns within each group.

ggplot2: theory and practice

ggplot2 is a system for creating graphics, based on the Grammar of Graphics.

Just like natural language has a grammar (nouns, verbs, adjectives), graphics have a grammar too:

• Data: What you want to visualize.
• Aesthetics (aes): How variables map to visual properties (x, y, color, size).
• Geometries (geom): The type of plot (points, lines, bars).
• Scales: Control how aesthetic mappings appear.
• Facets: Split into multiple subplots.
• Themes: Control non-data appearance (fonts, backgrounds).

Tip

`gplot builds plots by adding layers with the + operator.

Anatomy of a “ggplot”

Every ggplot needs, at minimum:

• Data: a dataframe or tibble.
• Aesthetic mappings: which variables map to which visual properties.
• Geometry: how to represent the data visually.

ggplot(data = mpg,                          # 1. Data
       aes(x = displ, y = hwy, color = class)) + # 2. Aesthetics
  geom_point() # 3. Geometry

Histograms

A histogram is a visualization of a single continuous, quantitative variable (e.g., income or temperature).

A histogram can be created with geom_histogram.

mpg %>%
  ggplot(aes(x = cty)) +
  geom_histogram()

💭 Check-in

What happens if you modify bins or binwidth?

Histograms

A histogram is a visualization of a single continuous, quantitative variable (e.g., income or temperature).

A histogram can be created with geom_histogram.

💭 Check-in

What happens if you modify bins or binwidth?

Histograms are very useful!

Histograms show important visual information about a distribution:

• Shape: is it symmetric, skewed, etc.?
• Center: Where is the “typical” value?
• Spread : How variable is the data?
• Outliers: Are there unusual values?

💭 Check-in

How does the skew of a distribution affect measures of central tendency such as the mean or median?

Histograms vs. density plots

A density plot is a smoothed alternative to a histogram, created using kernel density estimation (KDE).

A density plot can be created with geom_density.

ggplot(mpg, aes(x = cty)) + 
  geom_density(fill = "steelblue") 

Overlaying multiple distributions

One benefit of a density plot is that it’s easier to overlay multiple distributions on the same plot. The alpha parameter controls the opacity of each distribution.

mpg %>%
  filter(class %in% c("compact", "suv")) %>%
  ggplot(aes(x = cty, fill = class)) + 
  geom_density(alpha = .5) +
  labs(title = "City MPG: Compact vs. SUV", fill = "Car Type") +
  theme_minimal()

Scatterplots

A scatterplot shows the relationship between two continuous variables, where each point represents an observation.

A scatterplot can be created with geom_point.

mpg %>%
  ggplot(aes(x = cty, y = hwy)) + 
  geom_point() 

Adding layers to a scatterplot

We can further modify the color, size, and shape of individual dots.

mpg %>%
  ggplot(aes(x = cty, y = hwy, color = class, size = cyl, shape = drv)) + 
  geom_point(alpha = .5) 

Tip

Use categorical variables for shape, categorical or continuous variables for color, and ordinal or continuous variables for size.

Plotting a regression line

We can also use geom_smooth to plot a regression line (or another non-linear function) over our scatterplot.

(If there are multiple colors, etc., a different line will be plotted for each color.)

mpg %>%
  ggplot(aes(x = cty, y = hwy)) + 
  geom_smooth(method = "lm") +
  geom_point(alpha = .5) 

Bar plots

A barplot visualizes the relationship between one continuous variable and (at least one) categorical variable.

A barplot can be created with geom_bar.

• By default, geom_bar will count occurrences (like a histogram for categorical variables).
• You can also calculate values like the mean using geom_bar(stat = "summary").
• If you already have values computed (e.g., a mean or count), you can use geom_col or `geom_bar(stat = “identity”).

Bar plots: counts

By default, geom_bar will count the occurrences of each class.

mpg %>%
  ggplot(aes(x = drv)) +
  geom_bar() +
  theme_minimal()

💭 Check-in

Create a barplot showing the counts of some other categorical variable in the mpg dataframe.

Bar plots: summaries

You can also calculate summary statistics, i.e., the mean of some y variable for each level of the x variable.

Use reorder to reorder the bars in terms of their values.

mpg %>%
  ggplot(aes(x = reorder(drv, hwy), y = hwy)) +
  geom_bar(stat = "summary", fun = "mean") +
  theme_minimal()

Bar plots with group_by

Alternatively, you can calculate summary statistics using group_by %>% summarise, and pipe the output into a ggplot call.

mpg %>%
  group_by(drv) %>%
  summarise(mean_hwy = mean(hwy)) %>%
  ggplot(aes(x = reorder(drv, mean_hwy), y = mean_hwy)) +
  geom_bar(stat = "identity") +
  theme_minimal()

💭 Check-in

How would you instead plot the mean cty miles per gallon?

Error bars: stat_summary

Often, you want to display some measure of dispersion in addition to the mean. One approach is to use stat_summary.

mpg %>%
  ggplot(aes(x = reorder(drv, hwy), y = hwy)) +
  geom_bar(stat = "summary", fun = "mean") +
  stat_summary(fun.data = mean_se, geom = "errorbar", width = 0.2) +
  labs(x = "Drive Type", y = "Mean Highway MPG") +
  theme_minimal()

Error bars with group_by

Alternatively, with the group_by method, you can use first calculate the standard error, then pipe the result into geom_errorbar.

mpg %>%
  group_by(drv) %>%
  summarise(
    mean_hwy = mean(hwy),
    se_hwy = sd(hwy) / sqrt(n())
  ) %>%
  ggplot(aes(x = reorder(drv, mean_hwy), y = mean_hwy)) +
  geom_bar(stat = "identity") +
  geom_errorbar(aes(ymin = mean_hwy - se_hwy, 
                    ymax = mean_hwy + se_hwy),
                width = 0.2) +
  labs(x = "Drive Type", y = "Mean Highway MPG") +
  theme_minimal()

💭 Check-in

How would you plot a confidence interval for two standard errors?

Error bars with group_by

Alternatively, with the group_by method, you can use first calculate the standard error, then pipe the result into geom_errorbar.

💭 Check-in

How would you plot a confidence interval for two standard errors?

Bar plots with fill

You can add further information to a barplot using the fill parameter. Use position_dodge to show the bars side by side (rather than stacked).

mpg %>%
  ggplot(aes(x = reorder(drv, hwy), y = hwy, fill = class)) +
  geom_bar(stat = "summary", fun = "mean", position = position_dodge(width = 0.9)) +
  stat_summary(fun.data = mean_se, 
               geom = "errorbar", 
               position = position_dodge(width = 0.9), 
               width = 0.2) +
  theme_minimal()

Boxplots and violin plots

Boxplots and violin plots show more detailed information about underlying distribution for a given category.

• A boxplot shows the median, along with the inter-quartile range.
• A violinplot shows the full distribution as a density curve, rotated and mirrored.
• As with barplots, you can also modify the color of each box/violin.

Boxplots with geom_boxplot

mpg %>%
  ggplot(aes(x = reorder(drv, hwy), y = hwy)) +
  geom_boxplot() +
  theme_minimal()

Violin plots with geom_violin

mpg %>%
  ggplot(aes(x = reorder(drv, hwy), y = hwy)) +
  geom_violin() +
  theme_minimal()

Adding styling to a plot

Let’s revisit a plot we worked on earlier, with better labels and styling.

mpg %>%
  ggplot(aes(x = cty, y = hwy, color = drv, size = cyl, shape = drv)) + 
  geom_point(alpha = .5) +
  labs(x = "City mpg",
       y = "Highway mpg",
       size = "Number of cylinders",
       color = "Drive Train",
       shape = "Drive Train") +
  theme_minimal() +
  scale_color_viridis_d() +
  theme(
    axis.title = element_text(size = 16),
    legend.title = element_text(size = 12),
    legend.text = element_text(size = 10)
  )

Labeling specific points

Let’s revisit a plot we worked on earlier, with better labels and styling.

mpg_labeled <- mpg %>%
  filter(hwy > 42) %>%
  filter(class == "compact")

mpg %>%
  ggplot(aes(x = cty, y = hwy, color = drv, size = cyl, shape = drv)) + 
  geom_point(alpha = .5) +
  geom_text(data = mpg_labeled, 
            aes(label = model), 
            hjust = 1.2, vjust = 0, 
            size = 4, 
            color = "black",
            show.legend = FALSE) +
  theme_minimal(base_size = 14)

Other plotting packages

ggplot has a ton of useful functions and geom types, and it’s probably all you “need”——but there are other options too.

• ggcorrplot: gg-style correlation matrices.
• ggridges: gg-style “ridge” plots (density plots).

Using ggcorrplot

ggcorrplot is a library (and function) that visualizes a correlation matrix.

library(ggcorrplot)

cor_matrix = mpg %>%
  select(hwy, cty, displ, cyl) %>%
  cor()
ggcorrplot(cor_matrix)

Using ggridges

ggridges is a library for arranging density plots in a staggered fashion.

library(ggridges)
mpg %>%
  ggplot(aes(x = hwy, y = reorder(class, hwy), fill = drv)) +
  geom_density_ridges(alpha = .7, color = NA) +
  theme_ridges()

Summary

• Data visualization is central to CSS.
  • Crucial for exploring data, communicating insights, and impacting decisions.
• ggplot is a versatile and powerful library for creating clear, elegant figures.
• R also supports a number of additional libraries for visualization.
• The best way to learn to make visualizations is to make them!