此字符串在seaborn散点图的Palette参数中意味着什么？

Question

From seaborn gallery - scatter plots ,

import seaborn as sns
import matplotlib.pyplot as plt
sns.set(style="whitegrid")

# Load the example iris dataset
diamonds = sns.load_dataset("diamonds")

# Draw a scatter plot while assigning point colors and sizes to different
# variables in the dataset
f, ax = plt.subplots(figsize=(6.5, 6.5))
sns.despine(f, left=True, bottom=True)
clarity_ranking = ["I1", "SI2", "SI1", "VS2", "VS1", "VVS2", "VVS1", "IF"]
sns.scatterplot(x="carat", y="price",
                hue="clarity", size="depth",
                palette="ch:r=-.2,d=.3_r",
                hue_order=clarity_ranking,
                sizes=(1, 8), linewidth=0,
                data=diamonds, ax=ax)

What does this cryptic string "ch:r=-.2,d=.3_r" mean?

The only reference I can find is on seaborn.color_palette 's doc, saying that

 Other options: name of matplotlib cmap, 'ch:<cubehelix arguments>', 'hls', 'husl', or a list of colors in any format matplotlib accepts 

But still cannot find anything in matplotlib doc.

So, what does it mean exactly? So far I only know the suffix _r means 'reverse' of hues.

Answer 1

The "ch:r=-.2,d=.3_r" syntax is seaborn specific. So no wonder there is nothing about it in the matplotlib docs.

Possible options of <cubehelix arguments> in 'ch:<cubehelix arguments>' can implicitely be deduced from the seaborn.cubehelix_palette documentation.

This provides arguments like

 start : float, 0 <= start <= 3 The hue at the start of the helix. rot : float Rotations around the hue wheel over the range of the palette. gamma : float 0 <= gamma Gamma factor to emphasize darker (gamma < 1) or lighter (gamma > 1) colors. hue : float, 0 <= hue <= 1 Saturation of the colors. dark : float 0 <= dark <= 1 Intensity of the darkest color in the palette. light : float 0 <= light <= 1 Intensity of the lightest color in the palette. 

You can use those in the string like

"ch:rot=-0.2,dark=0.3"

and in order to shorten this, using only the first letter is sufficient

"ch:r=-0.2,d=0.3"

Note that it is not possible to select the number of colors through this string mini language.