How to Add a Secondary Y-Axis in ggplot2 (sec_axis)

0. Live Code: Dual Axis Plot

1. The Philosophy of Dual Axes in ggplot2

Unlike matplotlib (which allows arbitrary scale overlay using `twinx()`), ggplot2 strictly forces the secondary axis to be a transformation of the primary axis (e.g. multiplying by a constant scale factor). This maintains the underlying coordinate layout integrity.

2. Defining a Secondary Scale (sec_axis)

Map the primary variable. Then define `sec.axis` inside the primary scale function, indicating the mathematical formula:

# Secondary axis mapping (e.g. Celsius to Fahrenheit: * 1.8 + 32)
ggplot(df, aes(x = time, y = temp_celsius)) +
  geom_line() +
  scale_y_continuous(
    name = "Celsius (°C)",
    sec.axis = sec_axis(trans = ~ . * 1.8 + 32, name = "Fahrenheit (°F)")
  )

3. Correct Scaling Transformations

To plot two completely different units (e.g. Temperature 0–40 and Rain 0–200), transform the rain column values in the `geom` layer mapping, and apply the inverse scale transformation in `sec_axis`:

# Rain scale is 5x Temp scale. Shrink rain values to fit on temp axis.
ggplot(df, aes(x = day)) +
  geom_line(aes(y = temperature, color = "Temp")) +
  geom_col(aes(y = precipitation / 5, fill = "Rain"), alpha = 0.3) +
  scale_y_continuous(
    name = "Temperature (°C)",
    sec.axis = sec_axis(trans = ~ . * 5, name = "Precipitation (mm)")
  )

4. Alternative Designs: Panel Facets

If axes scaling represents completely unrelated entities, consider using `facet_wrap(..., scales = "free_y")` to stack plots vertically. This is often cleaner and avoids scaling confusion.

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