Position adjustments

Gears

Transmission type and number of gears are encoded in the same variable, a simple bar plot helps:

ggplot(data = mpg) +
  geom_bar(mapping = aes(x = trans))

Visual inspection reveals that it’s four gears for automatic, and five gears for manual transmission.

Discriminate by car class

ggplot(data = mpg) +
  geom_bar(mapping = aes(x = class, fill = trans))

SUVs have automatic mostly, 2-seaters favor six-gear manual, compact and subcompact mostly have five-gear manual transmission. This is much easier to see with position = "dodge", the different width of the bars indicates the number of different subgroups for each car class:

ggplot(data = mpg) +
  geom_bar(
    aes(x = class, fill = trans), 
    position = "dodge"
  )

Boxplot

I’m using liters per 100 km as measure for fuel economy here.

ggplot(data = mpg) +
  geom_boxplot(mapping = aes(x = drv, y = 235 / hwy))

Forward drivetrains seem much more economical. Does the number of cylinders play a role? I’ll try the “fill” aesthetic:

ggplot(data = mpg) +
  geom_boxplot(mapping = aes(x = drv, y = 235 / hwy, fill = cyl))

No dice. Do I also need group = ?

ggplot(data = mpg) +
  geom_boxplot(
    mapping = aes(
      x = drv,
      y = 235 / hwy,
      fill = cyl,
      group = cyl
    )
  )

The legend reveals that cyl is a continuous variable. I’ll use its categorical equivalent, because the range is very limited.

ggplot(data = mpg) +
  geom_boxplot(
    mapping = aes(
      x = drv,
      y = 235 / hwy,
      fill = factor(cyl)
    )
  )

The default position setting looks good, let’s try "identity":

ggplot(data = mpg) +
  geom_boxplot(
    mapping = aes(
      x = drv,
      y = 235 / hwy,
      fill = factor(cyl)
    ),
    position = "identity"
  )

Doesn’t look useful.