ggplot - 提取与连续色标中的中断相关的值

Question

I would like to extract the breaks and the colour values associated with a ggplot continuous colour scale. There are multiple answers to finding the colour associated with each date point (like this ), which can also be used to get discrete scale values, but I haven't seen an approach for a continuous colour scale. I don't want to force the scales, just retrieve the values that ggplot generates.

example:

library(ggplot)
df <- data.frame(x = 1:10, y = 1:10, col = 11:20)
ggplot(df) +
  geom_point(aes(x = x, y = y, colour = col))

I would like to get a data frame showing breaks (12.5, 15, 17.5, 20) and the colour values associated with them.

Many thanks!

Answer 1

There are two ways of doing this, once with building the plot and once without building the plot.

If we build the plot;

library(ggplot2)

df <- data.frame(x = 1:10, y = 1:10, col = 11:20)

ggplot(df) +
  geom_point(aes(x = x, y = y, colour = col))

We can extract the scale and use it to retrieve the relevant information.

# Using build plot
build <- ggplot_build(last_plot())

scale <- build$plot$scales$get_scales("colour")
breaks  <- scale$get_breaks()
colours <- scale$map(breaks)

data.frame(breaks = breaks, colours = colours)
#>   breaks colours
#> 1     NA  grey50
#> 2   12.5 #1D3F5E
#> 3   15.0 #2F638E
#> 4   17.5 #4289C1
#> 5   20.0 #56B1F7

Alternatively, we can skip building the plot and use the scales themselves directly, provided we 'train' the scales by showing it the limits of the data.

scale  <- scale_colour_continuous()
scale$train(range(df$col))
breaks  <- scale$get_breaks()
colours <- scale$map(breaks)

data.frame(breaks = breaks, colours = colours)

As you can see, the default breaks algorithm produces an out-of-bounds break. If you want to use the information later on, it might be good to filter those out.