Box plots in Pandas

Introduction

A box plot helps you see how data is spread out and if there are any unusual values.

To compare test scores of students in different classes.

To check the spread of daily temperatures over a month.

To find outliers in sales data for different products.

To summarize the distribution of heights in a group of people.

Syntax

Pandas

DataFrame.boxplot(column=None, by=None, grid=True, figsize=None, fontsize=None, rot=0, return_type=None, ax=None, **kwargs)

column selects which data column to plot.

by groups data by a column to make multiple box plots.

Examples

Draws a box plot for the 'Age' column.

Pandas

df.boxplot(column='Age')

Draws box plots of 'Salary' grouped by 'Department'.

Pandas

df.boxplot(column='Salary', by='Department')

Draws box plots for all numeric columns with a custom size and no grid.

Pandas

df.boxplot(figsize=(8,6), grid=False)

Sample Program

This code creates a small table with departments, salaries, and ages. It then draws box plots to compare salaries in each department. The plot shows the spread and any unusual salary values.

Pandas

import pandas as pd
import matplotlib.pyplot as plt

data = {'Department': ['HR', 'HR', 'IT', 'IT', 'Sales', 'Sales'],
        'Salary': [50000, 52000, 60000, 61000, 45000, 47000],
        'Age': [25, 28, 30, 35, 22, 24]}
df = pd.DataFrame(data)

# Draw box plots of Salary grouped by Department
df.boxplot(column='Salary', by='Department', grid=False, figsize=(6,4))
plt.suptitle('')  # Remove automatic title
plt.title('Salary Distribution by Department')
plt.ylabel('Salary')
plt.show()

Important Notes

Box plots show median, quartiles, and possible outliers.

Use plt.show() to display the plot when using pandas with matplotlib.

Grouping by a column helps compare different categories side by side.

Summary

Box plots help visualize data spread and outliers.

Use DataFrame.boxplot() to create box plots in pandas.

Grouping data by a column shows comparisons between groups.