Display the number of observations in a violinplot


It is a good practice to specify the number of observations for each group within a violinplot. Indeed, a group with 10 observations can have the exact same shape as a group with 10,000 observations, though they will be considered quite differently in statistical analysis.

In the following example, we start from a simple violinplot and add annotations to it.

To do so we:

  • calculate the median sepal_length for each group and store them in a variable named 'medians'
  • we then create a 'nobs' list which stores the number of observations for each group
  • eventually, we add labels to our figure.

To add labels, keep in mind that seaborn is built on top of matplotlib, thus seaborn objects can be stored in matplotlib axes or figures (here we store the violinplot in a matplotlib axes object named ax). This enables us to use matplotlib axes .get_xticklabels() as well as .text() functions and its various parameters (horizontalalignment, size, color, weight) to add text to our figure.

# libraries & dataset
import seaborn as sns
import matplotlib.pyplot as plt
import numpy as np
# set a grey background (use sns.set_theme() if seaborn version 0.11.0 or above) 
sns.set(style="darkgrid")
df = sns.load_dataset('iris')
 
# Basic violinplot stored in a matplotlib.axes object
ax = sns.violinplot(x="species", y="sepal_length", data=df)
 
# Calculate number of obs per group & median to position labels
medians = df.groupby(['species'])['sepal_length'].median().values
nobs = df['species'].value_counts().values
nobs = [str(x) for x in nobs.tolist()]
nobs = ["n: " + i for i in nobs]
 
# Add text to the figure
pos = range(len(nobs))
for tick, label in zip(pos, ax.get_xticklabels()):
   ax.text(pos[tick], medians[tick] + 0.03, nobs[tick],
            horizontalalignment='center',
            size='small',
            color='w',
            weight='semibold')
plt.show()

Violin

Density

Histogram

Boxplot

Ridgeline

Contact & Edit

👋 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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Violin

Density

Histogram

Boxplot

Ridgeline

Scatterplot

Heatmap

Correlogram

Bubble

Connected Scatter

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Barplot

Spider / Radar

Wordcloud

Parallel

Lollipop

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Stacked Area

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Animation with python

Animation

Cheat sheets

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3D