Seaborn — Statistical Visualization

Overview

Seaborn is a Python visualization library for creating publication-quality statistical graphics with minimal code. It works directly with pandas DataFrames, provides automatic statistical estimation (means, CIs, KDE), and offers attractive default themes. Built on matplotlib for full customization access.

When to Use

• Creating distribution plots (histograms, KDE, violin plots, box plots) for data exploration
• Visualizing relationships between variables with automatic trend fitting and confidence intervals
• Comparing distributions across categorical groups (treatment vs control, tissue types)
• Generating correlation heatmaps and clustered heatmaps
• Quick exploratory data analysis with pairplot for all pairwise relationships
• Multi-panel figures with automatic faceting by categorical variables
• For interactive plots with hover/zoom, use plotly instead
• For low-level figure control or custom layouts, use matplotlib directly

Prerequisites

pip install seaborn matplotlib pandas

Quick Start

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

df = sns.load_dataset("tips")
sns.scatterplot(data=df, x="total_bill", y="tip", hue="day", style="time")
plt.title("Tips by Day and Time")
plt.tight_layout()
plt.savefig("scatter.png", dpi=150)
print("Saved scatter.png")

Core API

1. Distribution Plots

Visualize univariate and bivariate distributions.

import seaborn as sns
import matplotlib.pyplot as plt

df = sns.load_dataset("tips")

# Histogram with density normalization
fig, axes = plt.subplots(1, 3, figsize=(15, 4))

sns.histplot(data=df, x="total_bill", hue="time", stat="density",
             multiple="stack", ax=axes[0])
axes[0].set_title("Histogram")

# KDE (smooth density estimate)
sns.kdeplot(data=df, x="total_bill", hue="time", fill=True,
            bw_adjust=0.8, ax=axes[1])
axes[1].set_title("KDE")

# ECDF (empirical cumulative distribution)
sns.ecdfplot(data=df, x="total_bill", hue="time", ax=axes[2])
axes[2].set_title("ECDF")

plt.tight_layout()
plt.savefig("distributions.png", dpi=150)
print("Saved distributions.png")

# Bivariate KDE with contours
sns.kdeplot(data=df, x="total_bill", y="tip", fill=True,
            levels=5, thresh=0.1, cmap="mako")
plt.title("Bivariate KDE")
plt.savefig("bivariate_kde.png", dpi=150)

2. Categorical Plots

Compare distributions or estimates across discrete categories.

import seaborn as sns
import matplotlib.pyplot as plt

df = sns.load_dataset("tips")
fig, axes = plt.subplots(1, 3, figsize=(15, 4))

# Box plot — quartiles and outliers
sns.boxplot(data=df, x="day", y="total_bill", hue="sex",
            dodge=True, ax=axes[0])
axes[0].set_title("Box Plot")

# Violin plot — KDE + quartiles
sns.violinplot(data=df, x="day", y="total_bill", hue="sex",
               split=True, inner="quart", ax=axes[1])
axes[1].set_title("Violin Plot")

# Bar plot — mean with CI
sns.barplot(data=df, x="day", y="total_bill", hue="sex",
            estimator="mean", errorbar="ci", ax=axes[2])
axes[2].set_title("Bar Plot (mean ± 95% CI)")

plt.tight_layout()
plt.savefig("categorical.png", dpi=150)
print("Saved categorical.png")

# Swarm plot — all individual observations, non-overlapping
sns.swarmplot(data=df, x="day", y="total_bill", hue="sex", dodge=True)
plt.title("Swarm Plot")
plt.savefig("swarm.png", dpi=150)

3. Relational Plots

Explore relationships between continuous variables.

import seaborn as sns
import matplotlib.pyplot as plt

df = sns.load_dataset("tips")

# Scatter with multiple semantic mappings
sns.scatterplot(data=df, x="total_bill", y="tip",
                hue="day", size="size", style="time")
plt.title("Scatter with Multi-Encoding")
plt.savefig("relational.png", dpi=150)

# Line plot with automatic aggregation and CI
fmri = sns.load_dataset("fmri")
sns.lineplot(data=fmri, x="timepoint", y="signal",
             hue="region", style="event", errorbar="sd")
plt.title("Line Plot (mean ± SD)")
plt.savefig("lineplot.png", dpi=150)

4. Regression Plots

Fit and visualize linear models.

import seaborn as sns
import matplotlib.pyplot as plt

df = sns.load_dataset("tips")

fig, axes = plt.subplots(1, 2, figsize=(12, 4))

# Linear regression with CI band
sns.regplot(data=df, x="total_bill", y="tip", ci=95, ax=axes[0])
axes[0].set_title("Linear Regression")

# Residual plot (check model assumptions)
sns.residplot(data=df, x="total_bill", y="tip", ax=axes[1])
axes[1].set_title("Residuals")

plt.tight_layout()
plt.savefig("regression.png", dpi=150)
print("Saved regression.png")

5. Matrix Plots

Visualize rectangular data (correlations, heatmaps).

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

# Correlation heatmap
df = sns.load_dataset("tips")
corr = df.select_dtypes(include=[np.number]).corr()

sns.heatmap(corr, annot=True, fmt=".2f", cmap="coolwarm",
            center=0, square=True, linewidths=0.5)
plt.title("Correlation Heatmap")
plt.tight_layout()
plt.savefig("heatmap.png", dpi=150)
print("Saved heatmap.png")

# Clustered heatmap with hierarchical clustering
flights = sns.load_dataset("flights").pivot(index="month", columns="year", values="passengers")
sns.clustermap(flights, cmap="viridis", standard_scale=1,
               figsize=(10, 8), linewidths=0.5)
plt.savefig("clustermap.png", dpi=150)

6. Figure-Level Functions and Faceting

Create multi-panel figures with automatic faceting.

import seaborn as sns

df = sns.load_dataset("tips")

# relplot — faceted scatter/line plots
g = sns.relplot(data=df, x="total_bill", y="tip",
                col="time", row="sex", hue="smoker",
                kind="scatter", height=3, aspect=1.2)
g.set_axis_labels("Total Bill ($)", "Tip ($)")
g.savefig("faceted_scatter.png", dpi=150)
print("Saved faceted_scatter.png")

# catplot — faceted categorical plots
g = sns.catplot(data=df, x="day", y="total_bill",
                col="time", kind="box", height=4, aspect=1)
g.set_titles("{col_name}")
g.savefig("faceted_boxplot.png", dpi=150)

7. Exploratory Grids (pairplot, jointplot)

Quickly explore all pairwise relationships.

import seaborn as sns

iris = sns.load_dataset("iris")

# Pairplot — matrix of pairwise relationships
g = sns.pairplot(iris, hue="species", corner=True,
                 diag_kind="kde", plot_kws={"alpha": 0.6})
g.savefig("pairplot.png", dpi=150)
print("Saved pairplot.png")

# Joint plot — bivariate + marginal distributions
g = sns.jointplot(data=iris, x="sepal_length", y="petal_length",
                  hue="species", kind="scatter")
g.savefig("jointplot.png", dpi=150)

Key Concepts

Figure-Level vs Axes-Level Functions

Understanding this distinction is critical for composing seaborn with matplotlib:

Feature	Axes-Level	Figure-Level
Examples	scatterplot, histplot, boxplot, heatmap	relplot, displot, catplot, lmplot
Returns	matplotlib.axes.Axes	FacetGrid / JointGrid / PairGrid
Faceting	Manual (create subplots yourself)	Built-in (col, row params)
Sizing	figsize on parent figure	height + aspect per subplot
Placement	ax= parameter	Cannot be placed in existing figure
Use when	Combining with other plot types, custom layouts	Quick faceted views, exploratory analysis

# Axes-level: embed in custom layout
fig, axes = plt.subplots(1, 2, figsize=(12, 5))
sns.boxplot(data=df, x="day", y="tip", ax=axes[0])
sns.scatterplot(data=df, x="total_bill", y="tip", ax=axes[1])

Data Format: Long vs Wide

Seaborn strongly prefers long-form (tidy) data where each variable is a column:

# Long-form (preferred) — works with all functions
#    subject  condition  value
# 0        1    control   10.5
# 1        1  treatment   12.3

# Wide-form — works with some functions (heatmap, lineplot)
#    control  treatment
# 0     10.5       12.3

# Convert wide → long
df_long = df.melt(var_name="condition", value_name="value")

Common Workflows

Workflow 1: Exploratory Data Analysis

Goal: Quickly survey a new dataset's distributions and relationships.

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

df = sns.load_dataset("penguins").dropna()

# 1. Pairwise relationships
g = sns.pairplot(df, hue="species", corner=True)
g.savefig("eda_pairplot.png", dpi=150)

# 2. Correlation heatmap
fig, ax = plt.subplots(figsize=(8, 6))
corr = df.select_dtypes(include=[np.number]).corr()
sns.heatmap(corr, annot=True, fmt=".2f", cmap="coolwarm", center=0, ax=ax)
ax.set_title("Feature Correlations")
plt.tight_layout()
plt.savefig("eda_corr.png", dpi=150)

# 3. Distribution by group
g = sns.displot(df, x="flipper_length_mm", hue="species",
                kind="kde", fill=True, col="sex", height=4)
g.savefig("eda_dist.png", dpi=150)
print("EDA figures saved")

Workflow 2: Publication-Quality Figure

Goal: Create a polished multi-panel figure for a paper.

import seaborn as sns
import matplotlib.pyplot as plt

sns.set_theme(style="ticks", context="paper", font_scale=1.1)
df = sns.load_dataset("penguins").dropna()

fig, axes = plt.subplots(1, 3, figsize=(12, 4))

# Panel A: Box plot
sns.boxplot(data=df, x="species", y="body_mass_g", hue="sex",
            palette="Set2", ax=axes[0])
axes[0].set_ylabel("Body Mass (g)")
axes[0].set_title("A", loc="left", fontweight="bold")

# Panel B: Scatter with regression
sns.regplot(data=df, x="flipper_length_mm", y="body_mass_g",
            scatter_kws={"alpha": 0.5, "s": 20}, ax=axes[1])
axes[1].set_xlabel("Flipper Length (mm)")
axes[1].set_ylabel("Body Mass (g)")
axes[1].set_title("B", loc="left", fontweight="bold")

# Panel C: Violin plot
sns.violinplot(data=df, x="species", y="bill_length_mm",
               inner="quart", palette="muted", ax=axes[2])
axes[2].set_ylabel("Bill Length (mm)")
axes[2].set_title("C", loc="left", fontweight="bold")

sns.despine(trim=True)
plt.tight_layout()
plt.savefig("figure_pub.pdf", dpi=300, bbox_inches="tight")
plt.savefig("figure_pub.png", dpi=300, bbox_inches="tight")
print("Publication figure saved as PDF and PNG")

Key Parameters

Parameter	Function	Default	Range / Options	Effect
hue	All plot functions	None	Column name	Color-encode a categorical/continuous variable
style	scatterplot, lineplot	None	Column name	Marker/line style encoding
size	scatterplot, lineplot	None	Column name	Point/line size encoding
col / row	Figure-level only	None	Column name	Create faceted subplots
col_wrap	Figure-level only	None	int	Max columns before wrapping
estimator	barplot, pointplot	"mean"	"mean", "median", callable	Aggregation function
errorbar	barplot, lineplot	("ci", 95)	"ci", "sd", "se", "pi"	Error bar type
stat	histplot	"count"	"count", "frequency", "density", "probability"	Histogram normalization
bw_adjust	kdeplot, violinplot	1.0	0.1–3.0	KDE bandwidth multiplier (higher=smoother)
multiple	histplot, kdeplot	"layer"	"layer", "stack", "dodge", "fill"	How to handle overlapping hue groups
kind	relplot, catplot, displot	varies	Plot type string	Select specific plot type for figure-level functions

Best Practices

1. Use DataFrames with named columns: Seaborn's strength is semantic mapping from column names. Avoid passing raw arrays — you lose axis labels and legend entries.

2. Choose axes-level for custom layouts, figure-level for faceting: If you need to combine different plot types in one figure, use axes-level functions with ax=. If you want automatic faceting, use figure-level functions.

3. Use set_theme() once at the start: Set style, context, and palette globally before creating plots. Reset with sns.set_theme().

4. Use "colorblind" palette for accessibility: sns.set_palette("colorblind") ensures your plots are distinguishable for readers with color vision deficiency.

5. Always call plt.tight_layout() before saving: Prevents axis labels from being clipped. For figure-level functions, use g.tight_layout().

6. Anti-pattern — using seaborn for highly customized layouts: If you need pixel-perfect control over every element, use matplotlib directly. Seaborn is for quick, attractive statistical plots, not for custom infographics.

7. Anti-pattern — wide-form data with semantic mappings: Functions like scatterplot(hue=...) require long-form data. Use pd.melt() to convert wide-form first.

Common Recipes

Recipe: Annotated Heatmap with Significance Stars

import seaborn as sns
import matplotlib.pyplot as plt
import numpy as np
from scipy import stats

# Compute correlation and p-values
df = sns.load_dataset("penguins").dropna().select_dtypes(include=[np.number])
n = len(df)
corr = df.corr()
p_values = df.corr().copy()
for i in df.columns:
    for j in df.columns:
        _, p = stats.pearsonr(df[i], df[j])
        p_values.loc[i, j] = p

# Create annotation with stars
annot = corr.round(2).astype(str)
for i in range(len(corr)):
    for j in range(len(corr)):
        if i != j and p_values.iloc[i, j] < 0.001:
            annot.iloc[i, j] += "***"
        elif i != j and p_values.iloc[i, j] < 0.01:
            annot.iloc[i, j] += "**"

sns.heatmap(corr, annot=annot, fmt="", cmap="coolwarm", center=0, square=True)
plt.title("Correlation with Significance")
plt.tight_layout()
plt.savefig("heatmap_sig.png", dpi=150)

Recipe: Custom PairGrid with Mixed Plot Types

import seaborn as sns

df = sns.load_dataset("penguins").dropna()
g = sns.PairGrid(df, hue="species", corner=True)
g.map_upper(sns.scatterplot, alpha=0.5)
g.map_lower(sns.kdeplot, fill=True, alpha=0.3)
g.map_diag(sns.histplot, kde=True)
g.add_legend()
g.savefig("custom_pairgrid.png", dpi=150)
print("Saved custom_pairgrid.png")

Troubleshooting

Problem	Cause	Solution
Legend outside plot area (clipped)	Figure-level functions place legend outside by default	Use g._legend.set_bbox_to_anchor((0.9, 0.5)) or plt.tight_layout()
Overlapping x-axis labels	Long category names	plt.xticks(rotation=45, ha="right") + plt.tight_layout()
Figure too small	Default sizing insufficient	Axes-level: fig, ax = plt.subplots(figsize=(10, 6)); Figure-level: height=6, aspect=1.5
Colors not distinct enough	Default palette has too-similar colors	Use sns.set_palette("bright") or sns.color_palette("husl", n_colors=N)
KDE too smooth or jagged	Bandwidth too wide or narrow	Adjust bw_adjust: lower (0.5) for detail, higher (2.0) for smoothing
FacetGrid cannot be placed in existing figure	Figure-level functions create their own figure	Use the corresponding axes-level function with ax= parameter
ValueError with hue on wide-form data	Semantic mappings require long-form	Convert with df.melt(var_name=..., value_name=...)

Related Skills

• matplotlib-scientific-plotting — low-level control, custom layouts, and publication-quality figure export
• plotly-interactive-visualization — interactive charts with hover, zoom, and HTML export
• statsmodels-statistical-modeling — statistical models whose results can be visualized with seaborn regression plots

References

• Seaborn documentation — official API reference and tutorial
• Seaborn gallery — visual examples of all plot types
• Waskom (2021) "seaborn: statistical data visualization" — JOSS