Zhenguo Zhang’s Blog /2025/05/10/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 😃

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