Scatterplots with three variables

Map to the color aesthetic

Simply insert color = <var> in the aes() call, where <var> is a variable in the mpg dataset:

ggplot(data = mpg) +
  geom_point(mapping = aes(x = displ, y = hwy, color = class))

ggplot(data = mpg) +
  geom_point(mapping = aes(x = displ, y = hwy, color = cyl))

ggplot(data = mpg) +
  geom_point(mapping = aes(x = displ, y = hwy, color = trans))

ggplot(data = mpg) +
  geom_point(mapping = aes(x = displ, y = hwy, color = drv))

Looks like the cars with large displacement are two-seaters with rear drivetrain.

Map to other aesthetics

The following won’t work, because cyl is stored as a continuous variable:

ggplot(data = mpg) +
  geom_point(mapping = aes(x = displ, y = hwy, shape = cyl))

## Error: A continuous variable can not be mapped to shape

The factor() function helps:

ggplot(data = mpg) +
  geom_point(mapping = aes(x = displ, y = hwy, shape = factor(cyl)))

On the other hand, a continuous variable is fine for the “size” aesthetic…

ggplot(data = mpg) +
  geom_point(mapping = aes(x = displ, y = hwy, size = cyl))

…which isn’t recommended (but still works) for a categorical variable:

ggplot(data = mpg) +
  geom_point(mapping = aes(x = displ, y = hwy, size = factor(cyl)))

## Warning: Using size for a discrete variable is not advised.

The “alpha” aesthetic controls transparency, and accepts both kinds of variable:

ggplot(data = mpg) +
  geom_point(mapping = aes(x = displ, y = hwy, alpha = cyl))

ggplot(data = mpg) +
  geom_point(mapping = aes(x = displ, y = hwy, alpha = factor(cyl)))

## Warning: Using alpha for a discrete variable is not advised.

Change more than one aesthetic

You can change any number of properties of a geom at your discretion, this is one of ggplot2’s strenghts.

ggplot(data = mpg) +
  geom_point(mapping = aes(
    x = displ,
    y = hwy,
    color = class,
    size = cyl,
    shape = drv,
    alpha = fl
  ))

## Warning: Using alpha for a discrete variable is not advised.

A legend is added automatically. It may not be as useful to change all properties at the same time, but it’s very easy to assign a variable to a different aesthetic, or use another variable for that aesthetic.

Map a variable to more than one aesthetic

There are no restrictions on how many times a variable is mapped in any given plot:

ggplot(data = mpg) +
  geom_point(mapping = aes(
    x = displ,
    y = hwy,
    color = displ,
    size = displ,
    alpha = displ
  ))

Mapping the same variable to two aesthetics is sometimes useful to further emphasize this variable.