Manual aesthetics

Blue transparent points

ggplot(data = mpg) +
  geom_point(
    mapping = aes(x = displ, y = hwy),
    alpha = 1 / 3,
    color = "blue"
  )

Some appear darker due to overplotting.

Assigning variable outside aes()

Fails with an error:

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

## Error in layer(data = data, mapping = mapping, stat = stat, geom = GeomPoint, : object 'year' not found

The error message is fairly confusing in some cases:

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

## Error in rep(value[[k]], length.out = n): attempt to replicate an object of type 'builtin'

Assigning constant inside aes()

This works, but creates a useless legend and may give unexpected results:

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

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

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

Valid values for manual aesthetics

• “color”: basic colors, numbers

• “alpha”: numbers between 0 and 1

• “shape”: integers from 0 to 25, negative integers seem to display a letter

• “size”: positive numbers