Mirror Density and Histogram Chart


It is a challange to compare the distribution of two variables in the same figure. This post shows how to tackle it with the mirror density and histogram charts using the seaborn and matplotlib libraries.

Mirror Density Chart

You can draw a basic density chart using the kdeplot() function of seaborn. The example below shows how to add a mirror density chart to your figure. It can be achieved using the lineplot() function with an input created by gaussian_kde(). You can simply multiply y axis values by -1 in order to reverse the chart:

# libraries
import numpy as np
from numpy import linspace
import pandas as pd
import seaborn as sns
import matplotlib.pyplot as plt
from scipy.stats import gaussian_kde

# dataframe
df = pd.DataFrame({
'var1': np.random.normal(size=1000),
'var2': np.random.normal(loc=2, size=1000) * -1
})

# Fig size
plt.rcParams["figure.figsize"]=12,8

# plot density chart for var1
sns.kdeplot(data=df, x="var1",  fill=True, alpha=1)

# plot density chart for var2
kde = gaussian_kde(df.var2)
x_range = linspace(min(df.var2), max(df.var2), len(df.var2))

# multiply by -1 to reverse axis (mirror plot)
sns.lineplot(x=x_range*-1, y=kde(x_range) * -1, color='orange') 
plt.fill_between(x_range*-1, kde(x_range) * -1, color='orange')

# add axis names        
plt.xlabel("value of x")
plt.axhline(y=0, linestyle='-',linewidth=1, color='black')



# show the graph
plt.show()

Mirror Histogram Chart

It is possible to apply the same technique using the histplot() and bar() functions to get a mirror histogram:

# libraries
import numpy as np
from numpy import linspace
import pandas as pd
import seaborn as sns
import matplotlib.pyplot as plt
from scipy.stats import gaussian_kde

# dataframe
df = pd.DataFrame({
'var1': np.random.normal(size=1000),
'var2': np.random.normal(loc=2, size=1000) * -1
})

# Fig size
plt.rcParams["figure.figsize"]=12,8

# plot histogram chart for var1
sns.histplot(x=df.var1, stat="density", bins=20)

# plot histogram chart for var2
n_bins = 20
# get positions and heights of bars
heights, bins = np.histogram(df.var2, density=True, bins=n_bins) 
# multiply by -1 to reverse it
heights *= -1
bin_width = np.diff(bins)[0]
bin_pos =( bins[:-1] + bin_width / 2) * -1

# plot
plt.bar(bin_pos, heights, width=bin_width, edgecolor='black')

# show the graph
plt.show()

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👋 This document is a work by Yan Holtz. Any feedback is highly encouraged. You can fill an issue on Github, drop me a message onTwitter, or send an email pasting yan.holtz.data with gmail.com.

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