Secondary axes: sec_axis

Mix.install([
  {:giggly, path: Path.join(__DIR__, "..")},
  {:kino, "~> 0.19"}
  ])

About this notebook

This notebook follows the examples on ggplot2's reference page for sec_axis(), which covers dup_axis() and derive() too, in the order the page gives them. Each example shows ggplot2's code first and then Giggly's. An example Giggly cannot draw yet keeps its ggplot2 code and names what it is waiting for, so the page is also a list of what remains.

A secondary axis is drawn opposite a position scale's primary axis and labels the same positions through a one-to-one transformation of the primary axis's values, such as a change of unit. sec_axis() describes one, given to a position scale's sec.axis argument; dup_axis() copies the primary axis; and derive(), given for any of a secondary axis's settings, takes that setting from the primary axis. Giggly has none of it yet: it has position scales, as Giggly.Plot.scale_y_continuous/2 makes, but no secondary axes.

A simple secondary axis

R's formula ~ . + 10 is a function of the primary axis's values, written .: each label on the right is 10 more than the one opposite it.

p <- ggplot(mtcars, aes(cyl, mpg)) +
  geom_point()

# Create a simple secondary axis
p + scale_y_continuous(sec.axis = sec_axis(~ . + 10))

Not in Giggly yet: sec_axis().

The primary axis's name

# Inherit the name from the primary axis
p + scale_y_continuous("Miles/gallon", sec.axis = sec_axis(~ . + 10, name = derive()))

Not in Giggly yet: sec_axis() and derive(), which gives the secondary axis the same title.

A copy of the primary axis

# Duplicate the primary axis
p + scale_y_continuous(sec.axis = dup_axis())

Not in Giggly yet: dup_axis().

A formula alone

# You can pass in a formula as a shorthand
p + scale_y_continuous(sec.axis = ~ .^2)

Not in Giggly yet: scale_y_continuous()'s sec.axis option, here a formula alone, squaring each value, as shorthand for sec_axis().

The time of day

df holds ten date-times four hours apart, from noon on 29 February 2012, each with a price.

# Secondary axes work for date and datetime scales too:
df <- data.frame(
  dx = seq(
    as.POSIXct("2012-02-29 12:00:00", tz = "UTC"),
    length.out = 10,
    by = "4 hour"
  ),
  price = seq(20, 200000, length.out = 10)
 )

# This may useful for labelling different time scales in the same plot
ggplot(df, aes(x = dx, y = price)) +
  geom_line() +
  scale_x_datetime(
    "Date",
    date_labels = "%b %d",
    date_breaks = "6 hour",
    sec.axis = dup_axis(
      name = "Time of Day",
      labels = scales::label_time("%I %p")
    )
  )

Not in Giggly yet: scale_x_datetime(), dup_axis() and scales::label_time(), here to label the date on the bottom axis and the time of day on the top.

Another time zone

Adding 8 * 3600 seconds to each time labels the top axis eight hours ahead, in GMT+8.

# or to transform axes for different timezones
ggplot(df, aes(x = dx, y = price)) +
  geom_line() +
  scale_x_datetime("
    GMT",
    date_labels = "%b %d %I %p",
    sec.axis = sec_axis(
      ~ . + 8 * 3600,
      name = "GMT+8",
      labels = scales::label_time("%b %d %I %p")
    )
  )

Not in Giggly yet: scale_x_datetime(), sec_axis() and scales::label_time().