[R] Use new_scale_xxx() function to add the same scale type in different ggplot layers

In one ggplot figure, normally you can only use one scale for each aesthetic mapping. For example, if you use scale_color_manual() to set the color scale for a layer, you cannot use another scale_color_manual() for another layer, or set the color scale more then once in the function aes(). However, you can use the new_scale_color() function from the ggnewscale package to add a new scale for the same aesthetic mapping in different layers.

In this post, I will showcase how to use the new_scale_color() function to add two different color scales in a ggplot figure. The first scale will be for a discrete variable (e.g., number of cylinders), and the second scale will be for a continuous variable (e.g., density level).

Load packages first.

library(ggplot2)
library(ggnewscale)

Use the mtcars dataset for the example

data(mtcars)

Create a plot with two color scales: 1. Points colored by ‘cyl’ (discrete) 2. Density contours colored by density level (continuous)

First, let’s make a scatter plot of mpg vs wt with points colored by the number of cylinders (cyl). We will use the geom_point() function for this layer.

plt <- ggplot(mtcars, aes(x = wt, y = mpg)) +
  # First layer: Scatter plot colored by cylinders (discrete variable)
  geom_point(aes(color = factor(cyl)), size = 3) +  
  scale_color_discrete(name = "Cylinders")

plt

Set new scale for the next layer

# Reset the color scale for the next layer
plt <- plt + new_scale_color()

Add a second layer: Density contours colored by density level (continuous variable)

plt <- plt +
  geom_density_2d(aes(color = after_stat(level))) +  
  scale_color_viridis_c(name = "Density Level", option = "magma") +
  
  # Add labels and theme
  labs(title = "Dual Color Scales with new_scale_color()",
       x = "Weight (1000 lbs)",
       y = "Miles per Gallon") +
  theme_minimal()

plt

Here I demonstrated how to use the new_scale_color() function from the ggnewscale package, one can also use new_scale_fill() for fill aesthetics. For other aesthetics, such as size, shape, etc., you can call new_scale("size"), new_scale("shape"), etc. to add new scales.

To learn more, check the webpage https://eliocamp.github.io/ggnewscale/

Happy programming 😃

Last modified on 2025-05-10