Understanding Matplotlib Axes Formatter with Seaborn Stripplot

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When working with matplotlib and seaborn, it’s often necessary to customize the appearance of your plots, including the x-axis labels. In this article, we’ll explore a common issue with using matplotlib.ticker.FuncFormatter on a seaborn stripplot.

Background Information

Matplotlib is a popular plotting library for Python that provides a wide range of visualization tools and techniques. Seaborn is built on top of matplotlib and extends its functionality to create informative and attractive statistical graphics.

Seaborn’s stripplot function is used to create striped plots, which are useful for displaying the distribution of individual data points in a dataset. The x-axis labels are automatically generated by seaborn, but they can be customized using various options, including the use of matplotlib.ticker.FuncFormatter.

Matplotlib Ticker FuncFormatter

A FuncFormatter is a function that formats a specific axis tick value according to a given format string. In the context of seaborn’s stripplot, we want to apply this formatting to the x-axis labels.

import matplotlib.pyplot as plt
import matplotlib.ticker as ticker

def my_formatter(x, pos):
    return f"{x:.2f}"

ax = plt.axes()
ax.xaxis.set_major_formatter(ticker.FuncFormatter(my_formatter))
plt.show()

In the above code, my_formatter is a function that takes two arguments: the tick value (x) and its position (pos). The formatted string is then returned.

The Problem

When we try to use a FuncFormatter with seaborn’s stripplot, it doesn’t work as expected. Instead of reformating the labels, the positions are changed.

import seaborn as sns
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

# Create sample data
df = pd.DataFrame({'x': np.random.randint(0,10,size=100) / 10, 'y': np.random.normal(size=100)})

# Create a figure with three subplots
fig, axarr = plt.subplots(3,1)

# First subplot: default formatting
sns.stripplot(data=df, x='x', y='y', ax=axarr[0])

# Second subplot: incorrect formatting
sns.stripplot(data=df, x='x', y='y', ax=axarr[1])
axarr[1].xaxis.set_major_formatter(plt.FuncFormatter(lambda x, pos: f'{x:.2f}'))
axarr[1].set_title('stripplot, incorrect formatting')
print(f'reformatted stripplot label: {axarr[1].get_xticklabels()[3]}')

# Third subplot: correct formatting
axarr[2].scatter(df.x, df.y, s=10, alpha=0.5)
axarr[2].xaxis.set_major_formatter(plt.FuncFormatter(lambda x, pos: f'{x:.2f}'))
axarr[2].set_title('scatterplot, correct formatting')
print(f'reformatted scatterplot label: {axarr[2].get_xticklabels()[3]}')

fig.tight_layout()

As shown in the example above, when we try to apply FuncFormatter to the second subplot’s x-axis labels, the positions change instead of being reformatted.

Solution

To fix this issue, we need to use a specific option on seaborn’s stripplot: native_scale=True. This parameter is new in Seaborn 0.12.0 and has been introduced for strip/swarm plots.

sns.stripplot(data=df, x='x', y='y', native_scale=True, ax=axarr[1])

With this option enabled, the x-axis labels are correctly reformatted according to the specified format string.

Conclusion

When working with seaborn’s stripplot, it’s essential to understand how FuncFormatter works and its limitations. By using the correct options, such as native_scale=True, we can ensure that our plots look professional and well-formatted.

Hugo Markdown

# References

*   Seaborn documentation: <https://seaborn.pydata.org/documentation.html>
*   Matplotlib documentation: <https://matplotlib.org/stable/tutorials/introductory/pyplot.html>

# Further Reading

For more information on matplotlib and seaborn, check out the following resources:

*   [Matplotlib Tutorial](https://matplotlib.org/stable/tutorials/index.html)
*   [Seaborn Tutorial](https://seaborn.pydata.org/tutorial.html)

Last modified on 2024-12-12