12.6 Using a Manually Defined Palette for a Continuous Variable

12.6.1 Problem

You want to use different colors for a continuous variable.

12.6.2 Solution

In the example here, we’ll specify the colors for a continuous variable using various gradient scales (Figure 12.12). The colors can be named, or they can be specified with RGB values:

library(gcookbook)  # Load gcookbook for the heightweight data set

# Create the base plot
hw_plot <- ggplot(heightweight, aes(x = ageYear, y = heightIn, colour = weightLb)) +
  geom_point(size = 3)

hw_plot

# A gradient with a white midpoint
library(scales)
hw_plot +
  scale_colour_gradient2(
    low = muted("red"),
    mid = "white",
    high = muted("blue"),
    midpoint = 110
  )

# With a gradient between two colors (black and white)
hw_plot +
  scale_colour_gradient(low = "black", high = "white")

# A gradient of n colors
hw_plot +
  scale_colour_gradientn(colours = c("darkred", "orange", "yellow", "white"))

Clockwise from top left: default colors, two-color gradient (black and white) with scale_colour_gradient(), three-color gradient with midpoint with scale_colour_gradient2(), four-color gradient with scale_colour_gradientn()

Figure 12.12: Clockwise from top left: default colors, two-color gradient (black and white) with scale_colour_gradient(), three-color gradient with midpoint with scale_colour_gradient2(), four-color gradient with scale_colour_gradientn()

For fill scales, use scale_fill_xxx() versions instead, where xxx is one of gradient, gradient2, or gradientn.

12.6.3 Discussion

Mapping continuous values to a color scale requires a continuously changing palette of colors. Table 12.2. lists the continuous color and fill scales.

Table 12.2: Continuous fill and color scales
Fill scale	Color scale	Description
`scale_fill_gradient()`	`scale_colour_gradient()`	Two-color gradient
`scale_fill_gradient2()`	`scale_colour_gradient2()`	Gradient with a middle color and two colors that diverge from it
`scale_fill_gradientn()`	`scale_colour_gradientn()`	Gradient with n colors, equally spaced
`scale_fill_viridis_c()`	`scale_colour_viridis_c()`	Viridis palettes

Notice that we used the muted() function in the examples. This is a function from the scales package that returns an RGB value that is a less-saturated version of the color chosen.

12.6.4 See Also

If you want use a discrete (categorical) scale instead of a continuous one, you can recode your data into categorical values. See Recipe 15.14.