Data Visualization

by anthropicsae1513ea94dcNo license27K starsListed Oct 8, 2026Updated Oct 8, 2026Repository updated today

Create effective data visualizations with Python (matplotlib, seaborn, plotly). Use when building charts, choosing the right chart type for a dataset, creating publication-quality figures, or applying design principles like accessibility and color theory.

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AI-generated overview

Guides chart selection and Python plotting code for clear, accessible data visualizations.

What it does
Provides chart-selection guidance mapping data relationships to suitable chart types, plus cautions on charts to avoid. Supplies Python code patterns for matplotlib, seaborn and plotly covering line, bar, histogram, heatmap and small-multiple charts, along with number formatting helpers. Also covers design principles for color, typography, layout and accuracy, and accessibility practices such as colorblind-safe palettes and alt text.
When to use it
Use when choosing an appropriate chart type for a dataset or relationship. Use when writing Python visualization code or producing publication-quality figures. Use when applying design, color or accessibility standards to charts.
Requirements
Instructions only; no scripts are shipped. The code patterns assume Python with matplotlib, seaborn, plotly, pandas and numpy available.

Data Visualization Skill

Chart selection guidance, Python visualization code patterns, design principles, and accessibility considerations for creating effective data visualizations.

Chart Selection Guide

Choose by Data Relationship

What You're ShowingBest ChartAlternatives
Trend over timeLine chartArea chart (if showing cumulative or composition)
Comparison across categoriesVertical bar chartHorizontal bar (many categories), lollipop chart
RankingHorizontal bar chartDot plot, slope chart (comparing two periods)
Part-to-whole compositionStacked bar chartTreemap (hierarchical), waffle chart
Composition over timeStacked area chart100% stacked bar (for proportion focus)
DistributionHistogramBox plot (comparing groups), violin plot, strip plot
Correlation (2 variables)Scatter plotBubble chart (add 3rd variable as size)
Correlation (many variables)Heatmap (correlation matrix)Pair plot
Geographic patternsChoropleth mapBubble map, hex map
Flow / processSankey diagramFunnel chart (sequential stages)
Relationship networkNetwork graphChord diagram
Performance vs. targetBullet chartGauge (single KPI only)
Multiple KPIs at onceSmall multiplesDashboard with separate charts

When NOT to Use Certain Charts

  • Pie charts: Avoid unless <6 categories and exact proportions matter less than rough comparison. Humans are bad at comparing angles. Use bar charts instead.
  • 3D charts: Never. They distort perception and add no information.
  • Dual-axis charts: Use cautiously. They can mislead by implying correlation. Clearly label both axes if used.
  • Stacked bar (many categories): Hard to compare middle segments. Use small multiples or grouped bars instead.
  • Donut charts: Slightly better than pie charts but same fundamental issues. Use for single KPI display at most.

Python Visualization Code Patterns

Setup and Style

python
import matplotlib.pyplot as pltimport matplotlib.ticker as mtickerimport seaborn as snsimport pandas as pdimport numpy as np
# Professional style setupplt.style.use('seaborn-v0_8-whitegrid')plt.rcParams.update({    'figure.figsize': (10, 6),    'figure.dpi': 150,    'font.size': 11,    'axes.titlesize': 14,    'axes.titleweight': 'bold',    'axes.labelsize': 11,    'xtick.labelsize': 10,    'ytick.labelsize': 10,    'legend.fontsize': 10,    'figure.titlesize': 16,})
# Colorblind-friendly palettesPALETTE_CATEGORICAL = ['#4C72B0', '#DD8452', '#55A868', '#C44E52', '#8172B3', '#937860']PALETTE_SEQUENTIAL = 'YlOrRd'PALETTE_DIVERGING = 'RdBu_r'

Line Chart (Time Series)

python
fig, ax = plt.subplots(figsize=(10, 6))
for label, group in df.groupby('category'):    ax.plot(group['date'], group['value'], label=label, linewidth=2)
ax.set_title('Metric Trend by Category', fontweight='bold')ax.set_xlabel('Date')ax.set_ylabel('Value')ax.legend(loc='upper left', frameon=True)ax.spines['top'].set_visible(False)ax.spines['right'].set_visible(False)
# Format dates on x-axisfig.autofmt_xdate()
plt.tight_layout()plt.savefig('trend_chart.png', dpi=150, bbox_inches='tight')

Bar Chart (Comparison)

python
fig, ax = plt.subplots(figsize=(10, 6))
# Sort by value for easy readingdf_sorted = df.sort_values('metric', ascending=True)
bars = ax.barh(df_sorted['category'], df_sorted['metric'], color=PALETTE_CATEGORICAL[0])
# Add value labelsfor bar in bars:    width = bar.get_width()    ax.text(width + 0.5, bar.get_y() + bar.get_height()/2,            f'{width:,.0f}', ha='left', va='center', fontsize=10)
ax.set_title('Metric by Category (Ranked)', fontweight='bold')ax.set_xlabel('Metric Value')ax.spines['top'].set_visible(False)ax.spines['right'].set_visible(False)
plt.tight_layout()plt.savefig('bar_chart.png', dpi=150, bbox_inches='tight')

Histogram (Distribution)

python
fig, ax = plt.subplots(figsize=(10, 6))
ax.hist(df['value'], bins=30, color=PALETTE_CATEGORICAL[0], edgecolor='white', alpha=0.8)
# Add mean and median linesmean_val = df['value'].mean()median_val = df['value'].median()ax.axvline(mean_val, color='red', linestyle='--', linewidth=1.5, label=f'Mean: {mean_val:,.1f}')ax.axvline(median_val, color='green', linestyle='--', linewidth=1.5, label=f'Median: {median_val:,.1f}')
ax.set_title('Distribution of Values', fontweight='bold')ax.set_xlabel('Value')ax.set_ylabel('Frequency')ax.legend()ax.spines['top'].set_visible(False)ax.spines['right'].set_visible(False)
plt.tight_layout()plt.savefig('histogram.png', dpi=150, bbox_inches='tight')

Heatmap

python
fig, ax = plt.subplots(figsize=(10, 8))
# Pivot data for heatmap formatpivot = df.pivot_table(index='row_dim', columns='col_dim', values='metric', aggfunc='sum')
sns.heatmap(pivot, annot=True, fmt=',.0f', cmap='YlOrRd',            linewidths=0.5, ax=ax, cbar_kws={'label': 'Metric Value'})
ax.set_title('Metric by Row Dimension and Column Dimension', fontweight='bold')ax.set_xlabel('Column Dimension')ax.set_ylabel('Row Dimension')
plt.tight_layout()plt.savefig('heatmap.png', dpi=150, bbox_inches='tight')

Small Multiples

python
categories = df['category'].unique()n_cats = len(categories)n_cols = min(3, n_cats)n_rows = (n_cats + n_cols - 1) // n_cols
fig, axes = plt.subplots(n_rows, n_cols, figsize=(5*n_cols, 4*n_rows), sharex=True, sharey=True)axes = axes.flatten() if n_cats > 1 else [axes]
for i, cat in enumerate(categories):    ax = axes[i]    subset = df[df['category'] == cat]    ax.plot(subset['date'], subset['value'], color=PALETTE_CATEGORICAL[i % len(PALETTE_CATEGORICAL)])    ax.set_title(cat, fontsize=12)    ax.spines['top'].set_visible(False)    ax.spines['right'].set_visible(False)
# Hide empty subplotsfor j in range(i+1, len(axes)):    axes[j].set_visible(False)
fig.suptitle('Trends by Category', fontsize=14, fontweight='bold', y=1.02)plt.tight_layout()plt.savefig('small_multiples.png', dpi=150, bbox_inches='tight')

Number Formatting Helpers

python
def format_number(val, format_type='number'):    """Format numbers for chart labels."""    if format_type == 'currency':        if abs(val) >= 1e9:            return f'${val/1e9:.1f}B'        elif abs(val) >= 1e6:            return f'${val/1e6:.1f}M'        elif abs(val) >= 1e3:            return f'${val/1e3:.1f}K'        else:            return f'${val:,.0f}'    elif format_type == 'percent':        return f'{val:.1f}%'    elif format_type == 'number':        if abs(val) >= 1e9:            return f'{val/1e9:.1f}B'        elif abs(val) >= 1e6:            return f'{val/1e6:.1f}M'        elif abs(val) >= 1e3:            return f'{val/1e3:.1f}K'        else:            return f'{val:,.0f}'    return str(val)
# Usage with axis formatterax.yaxis.set_major_formatter(mticker.FuncFormatter(lambda x, p: format_number(x, 'currency')))

Interactive Charts with Plotly

python
import plotly.express as pximport plotly.graph_objects as go
# Simple interactive line chartfig = px.line(df, x='date', y='value', color='category',              title='Interactive Metric Trend',              labels={'value': 'Metric Value', 'date': 'Date'})fig.update_layout(hovermode='x unified')fig.write_html('interactive_chart.html')fig.show()
# Interactive scatter with hover datafig = px.scatter(df, x='metric_a', y='metric_b', color='category',                 size='size_metric', hover_data=['name', 'detail_field'],                 title='Correlation Analysis')fig.show()

Design Principles

Color

  • Use color purposefully: Color should encode data, not decorate
  • Highlight the story: Use a bright accent color for the key insight; grey everything else
  • Sequential data: Use a single-hue gradient (light to dark) for ordered values
  • Diverging data: Use a two-hue gradient with neutral midpoint for data with a meaningful center
  • Categorical data: Use distinct hues, maximum 6-8 before it gets confusing
  • Avoid red/green only: 8% of men are red-green colorblind. Use blue/orange as primary pair

Typography

  • Title states the insight: "Revenue grew 23% YoY" beats "Revenue by Month"
  • Subtitle adds context: Date range, filters applied, data source
  • Axis labels are readable: Never rotated 90 degrees if avoidable. Shorten or wrap instead
  • Data labels add precision: Use on key points, not every single bar
  • Annotation highlights: Call out specific points with text annotations

Layout

  • Reduce chart junk: Remove gridlines, borders, backgrounds that don't carry information
  • Sort meaningfully: Categories sorted by value (not alphabetically) unless there's a natural order (months, stages)
  • Appropriate aspect ratio: Time series wider than tall (3:1 to 2:1); comparisons can be squarer
  • White space is good: Don't cram charts together. Give each visualization room to breathe

Accuracy

  • Bar charts start at zero: Always. A bar from 95 to 100 exaggerates a 5% difference
  • Line charts can have non-zero baselines: When the range of variation is meaningful
  • Consistent scales across panels: When comparing multiple charts, use the same axis range
  • Show uncertainty: Error bars, confidence intervals, or ranges when data is uncertain
  • Label your axes: Never make the reader guess what the numbers mean

Accessibility Considerations

Color Blindness

  • Never rely on color alone to distinguish data series
  • Add pattern fills, different line styles (solid, dashed, dotted), or direct labels
  • Test with a colorblind simulator (e.g., Coblis, Sim Daltonism)
  • Use the colorblind-friendly palette: sns.color_palette("colorblind")

Screen Readers

  • Include alt text describing the chart's key finding
  • Provide a data table alternative alongside the visualization
  • Use semantic titles and labels

General Accessibility

  • Sufficient contrast between data elements and background
  • Text size minimum 10pt for labels, 12pt for titles
  • Avoid conveying information only through spatial position (add labels)
  • Consider printing: does the chart work in black and white?

Accessibility Checklist

Before sharing a visualization:

  • Chart works without color (patterns, labels, or line styles differentiate series)
  • Text is readable at standard zoom level
  • Title describes the insight, not just the data
  • Axes are labeled with units
  • Legend is clear and positioned without obscuring data
  • Data source and date range are noted

Source and attribution

Source:anthropics/knowledge-work-pluginsindata/skills/data-visualizationat commitae1513e

License: No license

Content belongs to its original authors. SourceWeft indexes it from a public repository.

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