How to fill intervals under KDE curve with different colors

Question

I am looking for a way to color the intervals below the curve with different colors; on the interval x < 0, I would like to fill the area under the curve with one color and on the interval x >= 0 with another color, like the following image:

This is the code for basic kde plot:

fig, (ax1) = plt.subplots(1, 1, figsize = ((plot_size + 1.5) * 1,(plot_size + 1.5)))
sns.kdeplot(data=pd.DataFrame(w_contrast, columns=['contrast']), x="contrast", ax=ax1);
ax1.set_xlabel(f"Dry Yield Posterior Contrast (kg)");

Is there a way to fill the area under the curve with different colors using seaborn?

Asked By: Niko Gamulin

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Source

Answer 1

seaborn is a high level api for matplotlib, so the curve will have to be calculated; similar to, but simpler than this answer.
Calculate the values for the kde curve with scipy.stats.gaussian_kde
Use matplotlib.pyplot.fill_between to fill the areas.
Use scipy.integrate.simpson to calculate the area under the curve, which will be passed to matplotlib.pyplot.annotate to annotate.

import seaborn as sns
from scipy.stats import gaussian_kde
from scipy.integrate import simps
import numpy as np

# load sample data
df = sns.load_dataset('planets')

# create the kde model
kde = gaussian_kde(df.mass.dropna())

# plot
fig, ax = plt.subplots(figsize=(9, 6))
g = sns.kdeplot(data=df.mass, ax=ax, c='k')

# remove margins; optional
g.margins(x=0, y=0)

# get the min and max of the x-axis
xmin, xmax = g.get_xlim()

# create points between the min and max
x = np.linspace(xmin, xmax, 1000)

# calculate the y values from the model
kde_y = kde(x)

# select x values below 0
x0 = x[x < 0]

# get the len, which will be used for slicing the other arrays
x0_len = len(x0)

# slice the arrays
y0 = kde_y[:x0_len]
x1 = x[x0_len:]
y1 = kde_y[x0_len:]

# calculate the area under the curves
area0 = np.round(simps(y0, x0, dx=1) * 100, 0)
area1 = np.round(simps(y1, x1, dx=1) * 100, 0)

# fill the areas
g.fill_between(x=x0, y1=y0, color='r', alpha=.5)
g.fill_between(x=x1, y1=y1, color='b', alpha=.5)

# annotate
g.annotate(f'{area0:.0f}%', xy=(-1, 0.075), xytext=(10, 0.150), arrowprops=dict(arrowstyle="->", color='r', alpha=.5))
g.annotate(f'{area1:.0f}%', xy=(1, 0.05), xytext=(10, 0.125), arrowprops=dict(arrowstyle="->", color='b', alpha=.5))

Answered By: Trenton McKinney

How to fill intervals under KDE curve with different colors

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