# Chapter 5 Bivariate Graphs

One of the most fundamental questions in research is *“What is the relationship between A and B?”*. Bivariate graphs display the relationship between two variables. The type of graph will depend on the measurement level of each variable (categorical or quantitative).

## 5.1 Categorical vs. Categorical

When plotting the relationship between two categorical variables, stacked, grouped, or segmented bar charts are typically used. A less common approach is the mosaic chart (section 9.5).

In this section, we will look at automobile characteristics contained in mpg dataset that comes with the **ggplot2** package. It provides fuel efficiency data for 38 popular car models in 1998 and 2008 (see Appendix A.6).

### 5.1.1 Stacked bar chart

Let’s examine the relationship between automobile class and drive type (front-wheel, rear-wheel, or 4-wheel drive) for the automobiles in the mpg dataset.

```
library(ggplot2)
# stacked bar chart
ggplot(mpg, aes(x = class, fill = drv)) +
geom_bar(position = "stack")
```

From the Figure 5.1, we can see for example, that the most common vehicle is the SUV. All 2seater cars are rear wheel drive, while most, but not all SUVs are 4-wheel drive.

Stacked is the default, so the last line could have also been written as `geom_bar()`

.

### 5.1.2 Grouped bar chart

Grouped bar charts place bars for the second categorical variable side-by-side. To create a grouped bar plot use the `position = "dodge"`

option.

```
library(ggplot2)
# grouped bar plot
ggplot(mpg, aes(x = class, fill = drv)) +
geom_bar(position = "dodge")
```

Notice that all Minivans are front-wheel drive. By default, zero count bars are dropped and the remaining bars are made wider. This may not be the behavior you want. You can modify this using the `position = position_dodge(preserve = "single")"`

option.

```
library(ggplot2)
# grouped bar plot preserving zero count bars
ggplot(mpg, aes(x = class, fill = drv)) +
geom_bar(position = position_dodge(preserve = "single"))
```

Note that this option is only available in the later versions of `ggplot2`

.

### 5.1.3 Segmented bar chart

A segmented bar plot is a stacked bar plot where each bar represents 100 percent. You can create a segmented bar chart using the `position = "filled"`

option.

```
library(ggplot2)
# bar plot, with each bar representing 100%
ggplot(mpg, aes(x = class, fill = drv)) +
geom_bar(position = "fill") +
labs(y = "Proportion")
```

This type of plot is particularly useful if the goal is to compare the percentage of a category in one variable across each level of another variable. For example, the proportion of front-wheel drive cars go up as you move from compact, to midsize, to minivan.

### 5.1.4 Improving the color and labeling

You can use additional options to improve color and labeling. In the graph below

`factor`

modifies the order of the categories for the class variable and both the order and the labels for the drive variable

`scale_y_continuous`

modifies the y-axis tick mark labels

`labs`

provides a title and changed the labels for the x and y axes and the legend`scale_fill_brewer`

changes the fill color scheme

`theme_minimal`

removes the grey background and changed the grid color

```
library(ggplot2)
# bar plot, with each bar representing 100%,
# reordered bars, and better labels and colors
library(scales)
ggplot(mpg,
aes(x = factor(class,
levels = c("2seater", "subcompact",
"compact", "midsize",
"minivan", "suv", "pickup")),
fill = factor(drv,
levels = c("f", "r", "4"),
labels = c("front-wheel",
"rear-wheel",
"4-wheel")))) +
geom_bar(position = "fill") +
scale_y_continuous(breaks = seq(0, 1, .2),
label = percent) +
scale_fill_brewer(palette = "Set2") +
labs(y = "Percent",
fill="Drive Train",
x = "Class",
title = "Automobile Drive by Class") +
theme_minimal()
```

Each of these functions is discussed more fully in the section on Customizing graphs (see Section 11).

Next, let’s add percent labels to each segment. First, we’ll create a summary dataset that has the necessary labels.

```
# create a summary dataset
library(dplyr)
plotdata <- mpg %>%
group_by(class, drv) %>%
summarize(n = n()) %>%
mutate(pct = n/sum(n),
lbl = scales::percent(pct))
plotdata
```

```
## # A tibble: 12 × 5
## # Groups: class [7]
## class drv n pct lbl
## <chr> <chr> <int> <dbl> <chr>
## 1 2seater r 5 1 100%
## 2 compact 4 12 0.255 26%
## 3 compact f 35 0.745 74%
## 4 midsize 4 3 0.0732 7%
## 5 midsize f 38 0.927 93%
## 6 minivan f 11 1 100%
## 7 pickup 4 33 1 100%
## 8 subcompact 4 4 0.114 11%
## 9 subcompact f 22 0.629 63%
## 10 subcompact r 9 0.257 26%
## 11 suv 4 51 0.823 82%
## 12 suv r 11 0.177 18%
```

Next, we’ll use this dataset and the `geom_text`

function to add labels to each bar segment.

```
# create segmented bar chart
# adding labels to each segment
ggplot(plotdata,
aes(x = factor(class,
levels = c("2seater", "subcompact",
"compact", "midsize",
"minivan", "suv", "pickup")),
y = pct,
fill = factor(drv,
levels = c("f", "r", "4"),
labels = c("front-wheel",
"rear-wheel",
"4-wheel")))) +
geom_bar(stat = "identity",
position = "fill") +
scale_y_continuous(breaks = seq(0, 1, .2),
label = percent) +
geom_text(aes(label = lbl),
size = 3,
position = position_stack(vjust = 0.5)) +
scale_fill_brewer(palette = "Set2") +
labs(y = "Percent",
fill="Drive Train",
x = "Class",
title = "Automobile Drive by Class") +
theme_minimal()
```

Now we have a graph that is easy to read and interpret.

### 5.1.5 Other plots

Mosaic plots provide an alternative to stacked bar charts for displaying the relationship between categorical variables. They can also provide more sophisticated statistical information. See Section 9.5 for details.

## 5.2 Quantitative vs. Quantitative

The relationship between two quantitative variables is typically displayed using scatterplots and line graphs.

### 5.2.1 Scatterplot

The simplest display of two quantitative variables is a scatterplot, with each variable represented on an axis. Here, we will use the Salaries dataset described in Appendix A.1. First, let’s plot experience (*yrs.since.phd*) vs. academic salary (*salary*) for college professors.

```
library(ggplot2)
data(Salaries, package="carData")
# simple scatterplot
ggplot(Salaries,
aes(x = yrs.since.phd, y = salary)) +
geom_point()
```

As expected, salary tends to rise with experience, but the relationship may not be strictly linear. Note that salary appears to fall off after about 40 years of experience.

The `geom_point`

function has options that can be used to change the

`color`

- point color`size`

- point size`shape`

- point shape`alpha`

- point transparency. Transparency ranges from 0 (transparent) to 1 (opaque), and is a useful parameter when points overlap.

The functions `scale_x_continuous`

and `scale_y_continuous`

control the scaling on *x* and *y* axes respectively.

We can use these options and functions to create a more attractive scatterplot.

```
# enhanced scatter plot
ggplot(Salaries,
aes(x = yrs.since.phd, y = salary)) +
geom_point(color="cornflowerblue",
size = 2,
alpha=.8) +
scale_y_continuous(label = scales::dollar,
limits = c(50000, 250000)) +
scale_x_continuous(breaks = seq(0, 60, 10),
limits=c(0, 60)) +
labs(x = "Years Since PhD",
y = "",
title = "Experience vs. Salary",
subtitle = "9-month salary for 2008-2009")
```

Again, see Customizing graphs (Section 11) for more details.

#### 5.2.1.1 Adding best fit lines

It is often useful to summarize the relationship displayed in the scatterplot, using a best fit line. Many types of lines are supported, including linear, polynomial, and nonparametric (loess). By default, 95% confidence limits for these lines are displayed.

```
# scatterplot with linear fit line
ggplot(Salaries, aes(x = yrs.since.phd, y = salary)) +
geom_point(color= "steelblue") +
geom_smooth(method = "lm")
```

Clearly, salary increases with experience. However, there seems to be a dip at the right end - professors with significant experience, earning lower salaries. A straight line does not capture this non-linear effect. A line with a bend will fit better here.

A polynomial regression line provides a fit line of the form \[\hat{y} = \beta_{0} +\beta_{1}x + \beta{2}x^{2} + \beta{3}x^{3} + \beta{4}x^{4} + \dots\]

Typically either a quadratic (one bend), or cubic (two bends) line is used. It is rarely necessary to use a higher order( >3 ) polynomials. Adding a quadratic fit line to the salary dataset produces the following result.

```
# scatterplot with quadratic line of best fit
ggplot(Salaries, aes(x = yrs.since.phd, y = salary)) +
geom_point(color= "steelblue") +
geom_smooth(method = "lm",
formula = y ~ poly(x, 2),
color = "indianred3")
```

Finally, a smoothed nonparametric fit line can often provide a good picture of the relationship. The default in `ggplot2`

is a loess line which stands for for **lo**cally w**e**ighted **s**catterplot **s**moothing (Cleveland 1979).

```
# scatterplot with loess smoothed line
ggplot(Salaries, aes(x = yrs.since.phd, y = salary)) +
geom_point(color= "steelblue") +
geom_smooth(color = "tomato")
```

You can suppress the confidence bands by including the option `se = FALSE`

.

Here is a complete (and more attractive) plot.

```
# scatterplot with loess smoothed line
# and better labeling and color
ggplot(Salaries,
aes(x = yrs.since.phd, y = salary)) +
geom_point(color="cornflowerblue",
size = 2,
alpha=.6) +
geom_smooth(size = 1.5,
color = "darkgrey") +
scale_y_continuous(label = scales::dollar,
limits=c(50000, 250000)) +
scale_x_continuous(breaks = seq(0, 60, 10),
limits=c(0, 60)) +
labs(x = "Years Since PhD",
y = "",
title = "Experience vs. Salary",
subtitle = "9-month salary for 2008-2009") +
theme_minimal()
```

### 5.2.2 Line plot

When one of the two variables represents time, a line plot can be an effective method of displaying relationship. For example, the code below displays the relationship between time (*year*) and life expectancy (*lifeExp*) in the United States between 1952 and 2007. The data comes from the gapminder dataset (Appendix A.8).

```
data(gapminder, package="gapminder")
# Select US cases
library(dplyr)
plotdata <- filter(gapminder, country == "United States")
# simple line plot
ggplot(plotdata, aes(x = year, y = lifeExp)) +
geom_line()
```

It is hard to read indivial values in the graph above. In the next plot, we’ll add points as well.

```
# line plot with points
# and improved labeling
ggplot(plotdata, aes(x = year, y = lifeExp)) +
geom_line(size = 1.5,
color = "lightgrey") +
geom_point(size = 3,
color = "steelblue") +
labs(y = "Life Expectancy (years)",
x = "Year",
title = "Life expectancy changes over time",
subtitle = "United States (1952-2007)",
caption = "Source: http://www.gapminder.org/data/")
```

Time dependent data is covered in more detail under Time series (Section 8). Customizing line graphs is covered in the Customizing graphs (Section 11).

## 5.3 Categorical vs. Quantitative

When plotting the relationship between a categorical variable and a quantitative variable, a large number of graph types are available. These include bar charts using summary statistics, grouped kernel density plots, side-by-side box plots, side-by-side violin plots, mean/sem plots, ridgeline plots, and Cleveland plots. Each is considered in turn.

### 5.3.1 Bar chart (on summary statistics)

In previous sections, bar charts were used to display the number of cases by category for a single variable (Section 4.1.1) or for two variables (Section 5.1). You can also use bar charts to display other summary statistics (e.g., means or medians) on a quantitative variable for each level of a categorical variable.

For example, the following graph displays the mean salary for a sample of university professors by their academic rank.

```
data(Salaries, package="carData")
# calculate mean salary for each rank
library(dplyr)
plotdata <- Salaries %>%
group_by(rank) %>%
summarize(mean_salary = mean(salary))
# plot mean salaries
ggplot(plotdata, aes(x = rank, y = mean_salary)) +
geom_bar(stat = "identity")
```

We can make it more attractive with some options. In particular, the `factor`

function modifies the labels for each rank, the `scale_y_continuous`

function improves the y-axis labeling, and the `geom_text`

function adds the mean values to each bar.

```
# plot mean salaries in a more attractive fashion
library(scales)
ggplot(plotdata,
aes(x = factor(rank,
labels = c("Assistant\nProfessor",
"Associate\nProfessor",
"Full\nProfessor")),
y = mean_salary)) +
geom_bar(stat = "identity",
fill = "cornflowerblue") +
geom_text(aes(label = dollar(mean_salary)),
vjust = -0.25) +
scale_y_continuous(breaks = seq(0, 130000, 20000),
label = dollar) +
labs(title = "Mean Salary by Rank",
subtitle = "9-month academic salary for 2008-2009",
x = "",
y = "")
```

The `vjust`

parameter in the `geom_text`

function controls vertical justification and nudges the text above the bars. See Annotations (Section 11.7) for more details.

One limitation of such plots is that they do not display the distribution of the data - only the summary statistic for each group. The plots below correct this limitation to some extent.

### 5.3.2 Grouped kernel density plots

One can compare groups on a numeric variable by superimposing kernel density plots (Section 4.2.2) in a single graph.

```
# plot the distribution of salaries
# by rank using kernel density plots
ggplot(Salaries, aes(x = salary, fill = rank)) +
geom_density(alpha = 0.4) +
labs(title = "Salary distribution by rank")
```

The `alpha`

option makes the density plots partially transparent, so that we can see what is happening under the overlaps. Alpha values range from 0 (transparent) to 1 (opaque). The graph makes clear that, in general, salary goes up with rank. However, the salary range for full professors is *very* wide.

### 5.3.3 Box plots

A boxplot displays the 25^{th} percentile, median, and 75^{th} percentile of a distribution. The whiskers (vertical lines) capture roughly 99% of a normal distribution, and observations outside this range are plotted as points representing outliers (see the figure below).

Side-by-side box plots are very useful for comparing groups (i.e., the levels of a categorical variable) on a numerical variable.

```
# plot the distribution of salaries by rank using boxplots
ggplot(Salaries, aes(x = rank, y = salary)) +
geom_boxplot() +
labs(title = "Salary distribution by rank")
```

Notched boxplots provide an approximate method for visualizing whether groups differ. Although not a formal test, if the notches of two boxplots do not overlap, there is strong evidence (95% confidence) that the medians of the two groups differ (McGill, Tukey, and Larsen 1978).

```
# plot the distribution of salaries by rank using boxplots
ggplot(Salaries, aes(x = rank, y = salary)) +
geom_boxplot(notch = TRUE,
fill = "cornflowerblue",
alpha = .7) +
labs(title = "Salary distribution by rank")
```

In the example above, all three groups appear to differ.

One of the advantages of boxplots is that the width is usually not meaningful. This allows you to compare the distribution of many groups in a single graph.

### 5.3.4 Violin plots

Violin plots are similar to kernel density plots, but are mirrored and rotated 90^{o}.

```
# plot the distribution of salaries
# by rank using violin plots
ggplot(Salaries, aes(x = rank, y = salary)) +
geom_violin() +
labs(title = "Salary distribution by rank")
```

A violin plots capture more a a distribution’s shape than a boxplot, but does not indicate median or middle 50% of the data. A useful variation is to superimpose boxplots on violin plots.

```
# plot the distribution using violin and boxplots
ggplot(Salaries, aes(x = rank, y = salary)) +
geom_violin(fill = "cornflowerblue") +
geom_boxplot(width = .15,
fill = "orange",
outlier.color = "orange",
outlier.size = 2) +
labs(title = "Salary distribution by rank")
```

Be sure to set the `width`

parameter in the `geom_boxplot`

in order to assure the boxplots fit within the violin plots. You may need to play around with this in order to find a value that works well. Since geoms are layered, it is also important for the `geom_boxplot`

function to appear after the `geom_violin`

function. Otherwise the boxplots will be hidden beneath the violin plots.

### 5.3.5 Ridgeline plots

A ridgeline plot (also called a joyplot) displays the distribution of a quantitative variable for several groups. They’re similar to kernel density plots with vertical faceting, but take up less room. Ridgeline plots are created with the **ggridges** package.

Using the mpg dataset, let’s plot the distribution of city driving miles per gallon by car class.

```
# create ridgeline graph
library(ggplot2)
library(ggridges)
ggplot(mpg,
aes(x = cty, y = class, fill = class)) +
geom_density_ridges() +
theme_ridges() +
labs("Highway mileage by auto class") +
theme(legend.position = "none")
```

I’ve suppressed the legend here because it’s redundant (the distributions are already labeled on the *y*-axis). Unsurprisingly, pickup trucks have the poorest mileage, while subcompacts and compact cars tend to achieve ratings. However, there is a very wide range of gas mileage scores for these smaller cars.

Note the the possible overlap of distributions is the trade-off for a more compact graph. You can add transparency if the the overlap is severe using `geom_density_ridges(alpha = n)`

, where *n* ranges from 0 (transparent) to 1 (opaque). See the package vignette (https://cran.r-project.org/web/packages/ggridges/vignettes/introduction.html) for more details.

### 5.3.6 Mean/SEM plots

A popular method for comparing groups on a numeric variable is a mean plot with error bars. Error bars can represent standard deviations, standard errors of the means, or confidence intervals. In this section, we’ll calculate all three, but only plot means and standard errors to save space.

```
# calculate means, standard deviations,
# standard errors, and 95% confidence
# intervals by rank
library(dplyr)
plotdata <- Salaries %>%
group_by(rank) %>%
summarize(n = n(),
mean = mean(salary),
sd = sd(salary),
se = sd / sqrt(n),
ci = qt(0.975, df = n - 1) * sd / sqrt(n))
```

The resulting dataset is given below.

rank | n | mean | sd | se | ci |
---|---|---|---|---|---|

AsstProf | 67 | 80775.99 | 8174.113 | 998.6268 | 1993.823 |

AssocProf | 64 | 93876.44 | 13831.700 | 1728.9625 | 3455.056 |

Prof | 266 | 126772.11 | 27718.675 | 1699.5410 | 3346.322 |

```
# plot the means and standard errors
ggplot(plotdata,
aes(x = rank,
y = mean,
group = 1)) +
geom_point(size = 3) +
geom_line() +
geom_errorbar(aes(ymin = mean - se,
ymax = mean + se),
width = .1)
```

Although we plotted error bars representing the standard error, we could have plotted standard deviations or 95% confidence intervals. Simply replace `se`

with `sd`

or `error`

in the `aes`

option.

We can use the same technique to compare salary across rank and sex. (Technically, this is not bivariate since we’re plotting rank, sex, and salary, but it seems to fit here.)

```
# calculate means and standard errors by rank and sex
plotdata <- Salaries %>%
group_by(rank, sex) %>%
summarize(n = n(),
mean = mean(salary),
sd = sd(salary),
se = sd/sqrt(n))
# plot the means and standard errors by sex
ggplot(plotdata, aes(x = rank,
y = mean,
group=sex,
color=sex)) +
geom_point(size = 3) +
geom_line(size = 1) +
geom_errorbar(aes(ymin =mean - se,
ymax = mean+se),
width = .1)
```

Unfortunately, the error bars overlap. We can dodge the horizontal positions a bit to overcome this.

```
# plot the means and standard errors by sex (dodged)
pd <- position_dodge(0.2)
ggplot(plotdata,
aes(x = rank,
y = mean,
group=sex,
color=sex)) +
geom_point(position = pd,
size = 3) +
geom_line(position = pd,
size = 1) +
geom_errorbar(aes(ymin = mean - se,
ymax = mean + se),
width = .1,
position= pd)
```

Finally, lets add some options to make the graph more attractive.

```
# improved means/standard error plot
pd <- position_dodge(0.2)
ggplot(plotdata,
aes(x = factor(rank,
labels = c("Assistant\nProfessor",
"Associate\nProfessor",
"Full\nProfessor")),
y = mean, group=sex, color=sex)) +
geom_point(position=pd,
size=3) +
geom_line(position=pd,
size = 1) +
geom_errorbar(aes(ymin = mean - se,
ymax = mean + se),
width = .1,
position=pd,
size=1) +
scale_y_continuous(label = scales::dollar) +
scale_color_brewer(palette="Set1") +
theme_minimal() +
labs(title = "Mean salary by rank and sex",
subtitle = "(mean +/- standard error)",
x = "",
y = "",
color = "Gender")
```

This is a graph you could publish in a journal.

### 5.3.7 Strip plots

The relationship between a grouping variable and a numeric variable can be also displayed with a scatter plot. For example

```
# plot the distribution of salaries
# by rank using strip plots
ggplot(Salaries, aes(y = rank, x = salary)) +
geom_point() +
labs(title = "Salary distribution by rank")
```

These one-dimensional scatterplots are called strip plots. Unfortunately, overprinting of points makes interpretation difficult. The relationship is easier to see if the the points are jittered. Basically a small random number is added to each y-coordinate. To jitter the points, replace `geom_point`

with `geom_jitter`

.

```
# plot the distribution of salaries
# by rank using jittering
ggplot(Salaries, aes(y = rank, x = salary)) +
geom_jitter() +
labs(title = "Salary distribution by rank")
```

It is easier to compare groups if we use color.

```
# plot the distribution of salaries
# by rank using jittering
library(scales)
ggplot(Salaries,
aes(y = factor(rank,
labels = c("Assistant\nProfessor",
"Associate\nProfessor",
"Full\nProfessor")),
x = salary, color = rank)) +
geom_jitter(alpha = 0.7) +
scale_x_continuous(label = dollar) +
labs(title = "Academic Salary by Rank",
subtitle = "9-month salary for 2008-2009",
x = "",
y = "") +
theme_minimal() +
theme(legend.position = "none")
```

The option `legend.position = "none"`

is used to suppress the legend (which is not needed here). Jittered plots work well when the number of points in not overly large. Here, we can not only compare groups, but see the salaries of each individual faculty member. As a college professor myself, I want to know who is making more than $200,000 on a nine month contract!

Finally, we can superimpose boxplots on the jitter plots.

```
# plot the distribution of salaries
# by rank using jittering
library(scales)
ggplot(Salaries,
aes(x = factor(rank,
labels = c("Assistant\nProfessor",
"Associate\nProfessor",
"Full\nProfessor")),
y = salary, color = rank)) +
geom_boxplot(size=1,
outlier.shape = 1,
outlier.color = "black",
outlier.size = 3) +
geom_jitter(alpha = 0.5,
width=.2) +
scale_y_continuous(label = dollar) +
labs(title = "Academic Salary by Rank",
subtitle = "9-month salary for 2008-2009",
x = "",
y = "") +
theme_minimal() +
theme(legend.position = "none") +
coord_flip()
```

Several options were added to create this plot.

For the boxplot

`size = 1`

makes the lines thicker

`outlier.color = "black"`

makes outliers black`outlier.shape = 1`

specifies circles for outliers`outlier.size = 3`

increases the size of the outlier symbol

For the jitter

`alpha = 0.5`

makes the points more transparent`width = .2`

decreases the amount of jitter (.4 is the default)

Finally, the *x* and *y* axes are revered using the `coord_flip`

function (i.e., the graph is turned on its side).

Before moving on, it is worth mentioning the `geom_boxjitter`

function provided in the **ggpol** package. It creates a hybrid boxplot - half boxplot, half scaterplot.

```
# plot the distribution of salaries
# by rank using jittering
library(ggpol)
library(scales)
ggplot(Salaries,
aes(x = factor(rank,
labels = c("Assistant\nProfessor",
"Associate\nProfessor",
"Full\nProfessor")),
y = salary,
fill=rank)) +
geom_boxjitter(color="black",
jitter.color = "darkgrey",
errorbar.draw = TRUE) +
scale_y_continuous(label = dollar) +
labs(title = "Academic Salary by Rank",
subtitle = "9-month salary for 2008-2009",
x = "",
y = "") +
theme_minimal() +
theme(legend.position = "none")
```

Choose the approach that you find most useful.

### 5.3.8 Cleveland Dot Charts

Cleveland plots are useful when you want to compare each observation on a numeric variable, or compare a large number of groups on a numeric summary statistic. For example, say that you want to compare the 2007 life expectancy for Asian country using the gapminder dataset.

```
data(gapminder, package="gapminder")
# subset Asian countries in 2007
library(dplyr)
plotdata <- gapminder %>%
filter(continent == "Asia" &
year == 2007)
# basic Cleveland plot of life expectancy by country
ggplot(plotdata,
aes(x= lifeExp, y = country)) +
geom_point()
```

Comparisons are usually easier if the *y*-axis is sorted.

```
# Sorted Cleveland plot
ggplot(plotdata, aes(x=lifeExp,
y=reorder(country, lifeExp))) +
geom_point()
```

The difference in life expectancy between countries like Japan and Afghanistan is striking.

Finally, we can use options to make the graph more attractive by removing unnecessary elements, like the grey background panel and horizontal reference lines, and adding a line segment connecting each point to the y axis.

```
# Fancy Cleveland plot
ggplot(plotdata, aes(x=lifeExp,
y=reorder(country, lifeExp))) +
geom_point(color="blue", size = 2) +
geom_segment(aes(x = 40,
xend = lifeExp,
y = reorder(country, lifeExp),
yend = reorder(country, lifeExp)),
color = "lightgrey") +
labs (x = "Life Expectancy (years)",
y = "",
title = "Life Expectancy by Country",
subtitle = "GapMinder data for Asia - 2007") +
theme_minimal() +
theme(panel.grid.major = element_blank(),
panel.grid.minor = element_blank())
```

This last plot is also called a lollipop graph (I wonder why?).

### References

*Journal of the American Statistical Association*74 (368): 829–36. https://doi.org/10.1080/01621459.1979.10481038.

*The American Statistician*32 (1): 12–16. https://doi.org/10.2307/2683468.