Is it possible to annotate a seaborn violin plot with number of observations in each group?

Question

I would like to annotate my violin plot with the number of observations in each group. So the question is essentially the same as this one, except:

• python instead of R,
• seaborn instead of ggplot, and
• violin plots instead of boxplots

Lets take this example from Seaborn API documentation:

import seaborn as sns
sns.set_style("whitegrid")
tips = sns.load_dataset("tips")
ax = sns.violinplot(x="day", y="total_bill", data=tips)

I'd like to have n=62, n=19, n=87, and n=76 on top of the violins. Is this doable?

Answer 1

In this situation, I like to precompute the annotated values and incorporate them into the categorical axis. In other words, precompute e.g., "Thurs, N = xxx"

That looks like this:

import seaborn as sns
sns.set_style("whitegrid")
ax= (
    sns.load_dataset("tips")
       .assign(count=lambda df: df['day'].map(df.groupby(by=['day'])['total_bill'].count()))
       .assign(grouper=lambda df: df['day'].astype(str) + '\nN = ' + df['count'].astype(str))
       .sort_values(by='day') 
       .pipe((sns.violinplot, 'data'), x="grouper", y="total_bill")
       .set(xlabel='Day of the Week', ylabel='Total Bill (USD)')   
)

Answer 2

You first need to store all values of y positions and x positions (using your dataset for that) in order to use ax.text, then a simple for loop can write everything in the positions desired:

import seaborn as sns
import matplotlib.pyplot as plt

tips = sns.load_dataset("tips")
ax = sns.violinplot(x="day", y="total_bill", data=tips)

yposlist = tips.groupby(['day'])['total_bill'].median().tolist()
xposlist = range(len(yposlist))
stringlist = ['n = 62','n = 19','n = 87','n = 76']

for i in range(len(stringlist)):
    ax.text(xposlist[i], yposlist[i], stringlist[i])

plt.show()

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