# 7.5: Plots with Two Variables

- Page ID
- 8743

Let’s check out mileage by car manufacturer. We’ll plot one *continuous* variable by one *nominal* one.

First, let’s make a bar plot by choosing the stat “summary” and picking the “mean” function to summarize the data.

```
ggplot(mpg, aes(manufacturer, hwy)) +
geom_bar(stat = "summary", fun.y = "mean") +
ylab('Highway mileage')
```

One problem with this plot is that it’s hard to read some of the labels because they overlap. How could we fix that? Hint: search the web for “ggplot rotate x axis labels” and add the appropriate command.

*TBD: fix*

```
ggplot(mpg, aes(manufacturer, hwy)) +
geom_bar(stat = "summary", fun.y = "mean") +
ylab('Highway mileage')
```

# 7.5.1 Adding on variables

What if we wanted to add another variable into the mix? Maybe the *year* of the car is also important to consider. We have a few options here. First, you could map the variable to another **aesthetic**.

```
# first, year needs to be converted to a factor
mpg$year <- factor(mpg$year)
ggplot(mpg, aes(manufacturer, hwy, fill = year)) +
geom_bar(stat = "summary", fun.y = "mean")
```

By default, the bars are *stacked* on top of one another. If you want to separate them, you can change the `position`

argument form its default to “dodge”.

```
ggplot(mpg, aes(manufacturer, hwy, fill=year)) +
geom_bar(stat = "summary",
fun.y = "mean",
position = "dodge")
```

```
ggplot(mpg, aes(year, hwy,
group=manufacturer,
color=manufacturer)) +
geom_line(stat = "summary", fun.y = "mean")
```

For a less visually cluttered plot, let’s try **facetting**. This creates *subplots* for each value of the `year`

variable.

```
ggplot(mpg, aes(manufacturer, hwy)) +
# split up the bar plot into two by year
facet_grid(year ~ .) +
geom_bar(stat = "summary",
fun.y = "mean")
```

# 7.5.2 Plotting dispersion

Instead of looking at just the means, we can get a sense of the entire distribution of mileage values for each manufacturer.

## 7.5.2.1 Box plot

```
ggplot(mpg, aes(manufacturer, hwy)) +
geom_boxplot()
```

A **box plot** (or box and whiskers plot) uses quartiles to give us a sense of spread. The thickest line, somewhere inside the box, represents the *median*. The upper and lower bounds of the box (the *hinges*) are the first and third quartiles (can you use them to approximate the interquartile range?). The lines extending from the hinges are the remaining data points, excluding **outliers**, which are plotted as individual points.

## 7.5.2.2 Error bars

Now, let’s do something a bit more complex, but much more useful – let’s create our own summary of the data, so we can choose which summary statistic to plot and also compute a measure of dispersion of our choosing.

```
# summarise data
mpg_summary <- mpg %>%
group_by(manufacturer) %>%
summarise(n = n(),
mean_hwy = mean(hwy),
sd_hwy = sd(hwy))
# compute confidence intervals for the error bars
# (we'll talk about this later in the course!)
limits <- aes(
# compute the lower limit of the error bar
ymin = mean_hwy - 1.96 * sd_hwy / sqrt(n),
# compute the upper limit
ymax = mean_hwy + 1.96 * sd_hwy / sqrt(n))
# now we're giving ggplot the mean for each group,
# instead of the datapoints themselves
ggplot(mpg_summary, aes(manufacturer, mean_hwy)) +
# we set stat = "identity" on the summary data
geom_bar(stat = "identity") +
# we create error bars using the limits we computed above
geom_errorbar(limits, width=0.5)
```

Error bars don’t always mean the same thing – it’s important to determine whether you’re looking at e.g. standard error or confidence intervals (which we’ll talk more about later in the course).

### 7.5.2.2.1 Minimizing non-data ink

The plot we just created is nice and all, but it’s tough to look at. The bar plots add a lot of ink that doesn’t help us compare engine sizes across manufacturers. Similarly, the width of the error bars doesn’t add any information. Let’s tweak which *geometry* we use, and tweak the appearance of the error bars.

```
ggplot(mpg_summary, aes(manufacturer, mean_hwy)) +
# switch to point instead of bar to minimize ink used
geom_point() +
# remove the horizontal parts of the error bars
geom_errorbar(limits, width = 0)
```

Looks a lot cleaner, but our points are all over the place. Let’s make a final tweak to make *learning something* from this plot a bit easier.

```
mpg_summary_ordered <- mpg_summary %>%
mutate(
# we sort manufacturers by mean engine size
manufacturer = reorder(manufacturer, -mean_hwy)
)
ggplot(mpg_summary_ordered, aes(manufacturer, mean_hwy)) +
geom_point() +
geom_errorbar(limits, width = 0)
```

# 7.5.3 Scatter plot

When we have multiple *continuous* variables, we can use points to plot each variable on an axis. This is known as a **scatter plot**. You’ve seen this example in your reading.

```
ggplot(mpg, aes(displ, hwy)) +
geom_point()
```

## 7.5.3.1 Layers of data

We can add layers of data onto this graph, like a *line of best fit*. We use a geometry known as a **smooth** to accomplish this.

```
ggplot(mpg, aes(displ, hwy)) +
geom_point() +
geom_smooth(color = "black")
```

We can add on points and a smooth line for another set of data as well (efficiency in the city instead of on the highway).

```
ggplot(mpg) +
geom_point(aes(displ, hwy), color = "grey") +
geom_smooth(aes(displ, hwy), color = "grey") +
geom_point(aes(displ, cty), color = "limegreen") +
geom_smooth(aes(displ, cty), color = "limegreen")
```