| name | data-visualization |
| description | 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. |
| user-invocable | false |
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 Showing | Best Chart | Alternatives |
|---|
| Trend over time | Line chart | Area chart (if showing cumulative or composition) |
| Comparison across categories | Vertical bar chart | Horizontal bar (many categories), lollipop chart |
| Ranking | Horizontal bar chart | Dot plot, slope chart (comparing two periods) |
| Part-to-whole composition | Stacked bar chart | Treemap (hierarchical), waffle chart |
| Composition over time | Stacked area chart | 100% stacked bar (for proportion focus) |
| Distribution | Histogram | Box plot (comparing groups), violin plot, strip plot |
| Correlation (2 variables) | Scatter plot | Bubble chart (add 3rd variable as size) |
| Correlation (many variables) | Heatmap (correlation matrix) | Pair plot |
| Geographic patterns | Choropleth map | Bubble map, hex map |
| Flow / process | Sankey diagram | Funnel chart (sequential stages) |
| Relationship network | Network graph | Chord diagram |
| Performance vs. target | Bullet chart | Gauge (single KPI only) |
| Multiple KPIs at once | Small multiples | Dashboard 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
import matplotlib.pyplot as plt
import matplotlib.ticker as mticker
import seaborn as sns
import pandas as pd
import numpy as np
plt.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,
})
PALETTE_CATEGORICAL = ['#4C72B0', '#DD8452', '#55A868', '#C44E52', '#8172B3', '#937860']
PALETTE_SEQUENTIAL = 'YlOrRd'
PALETTE_DIVERGING = 'RdBu_r'
Line Chart (Time Series)
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)
fig.autofmt_xdate()
plt.tight_layout()
plt.savefig('trend_chart.png', dpi=150, bbox_inches='tight')
Bar Chart (Comparison)
fig, ax = plt.subplots(figsize=(10, 6))
df_sorted = df.sort_values('metric', ascending=True)
bars = ax.barh(df_sorted['category'], df_sorted['metric'], color=PALETTE_CATEGORICAL[0])
for 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)
fig, ax = plt.subplots(figsize=(10, 6))
ax.hist(df['value'], bins=30, color=PALETTE_CATEGORICAL[0], edgecolor='white', alpha=0.8)
mean_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
fig, ax = plt.subplots(figsize=(10, 8))
pivot = 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
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)
for 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
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)
ax.yaxis.set_major_formatter(mticker.FuncFormatter(lambda x, p: format_number(x, 'currency')))
Interactive Charts with Plotly
import plotly.express as px
import plotly.graph_objects as go
fig = 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()
fig = 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:
中文字体配置(macOS)
matplotlib 默认 Arial 不支持中文,会渲染成方框。关键陷阱:plt.style.use(...) 会覆盖 rcParams 里的 font 设置,所以必须先 use style 再设置字体。
import matplotlib
import matplotlib.pyplot as plt
plt.style.use('seaborn-v0_8-whitegrid')
matplotlib.rcParams['font.sans-serif'] = ['Heiti TC', 'PingFang HK', 'Songti SC', 'Yuanti SC']
matplotlib.rcParams['font.family'] = 'sans-serif'
matplotlib.rcParams['axes.unicode_minus'] = False
macOS 上确认可用的中文字体(实测):Heiti TC、PingFang HK、Songti SC、Yuanti SC、STHeiti、Hiragino Sans GB。
注意:PingFang SC 在某些系统上不存在,要用 PingFang HK 替代。
排查字体可用性:
import matplotlib.font_manager as fm
fonts = sorted(set(f.name for f in fm.fontManager.ttflist))
print([f for f in fonts if any(k in f for k in ['Hei','Pin','Song','STH','Hira','Yu'])])
洞察卡片模式(KPI Summary Card)
数据图回答「数据是什么样」,但用户往往还需要「所以呢」—— 配一张文字版洞察卡作为同尺寸搭档,可以单独发也可以拼起来。
适用场景:研究报告首页、社交媒体配图、汇报展示、给非技术 stakeholder 看。
设计原则
- 每个洞察一个卡片:编号 + 标题 + 大数字 KPI + KPI 说明 + 一两行解读
- KPI 用大字号(≥24pt)+ 强色,是视觉锚点
- 标题用一句话陈述结论("销量极度集中在头部 10%"),不要用维度名("销量分布")
- 底部一行最终结论,深色块包住,作为整张卡的钉子
- 配色用左侧色条区分类别:紫=中性、红=警示、绿=正向
代码模板(4 卡片 + 结论条)
import matplotlib.pyplot as plt
import matplotlib.patches as patches
fig, ax = plt.subplots(figsize=(16, 11), dpi=150)
ax.set_xlim(0, 100); ax.set_ylim(0, 100); ax.axis('off')
fig.patch.set_facecolor('white')
INK, SUB = '#1a1f36', '#697386'
ACCENT, DANGER, SUCCESS = '#635bff', '#df1b41', '#00875a'
BG_CARD, LINE = '#f6f9fc', '#e3e8ee'
ax.text(50, 95, '主标题', ha='center', fontsize=24, fontweight='bold', color=INK)
ax.text(50, 91, '副标题:数据来源/时间范围', ha='center', fontsize=11, color=SUB)
ax.plot([15, 85], [88, 88], color=LINE, linewidth=1)
cards = [
(4, 50, 46, 32, '①', ACCENT, '标题陈述结论', '57%', 'KPI 说明', '正文一两行解读'),
]
for x, y, w, h, badge, color, title, kpi, kpi_label, body in cards:
ax.add_patch(patches.FancyBboxPatch((x, y), w, h,
boxstyle="round,pad=0.5,rounding_size=1.2",
linewidth=0, facecolor=BG_CARD, zorder=1))
ax.add_patch(patches.Rectangle((x+0.5, y+1), 0.6, h-2,
facecolor=color, zorder=2))
ax.text(x+3.5, y+h-4, badge, fontsize=22, fontweight='bold', color=color, va='top')
ax.text(x+9, y+h-4.5, title, fontsize=14, fontweight='bold', color=INK, va='top')
ax.text(x+3.5, y+h-13, kpi, fontsize=28, fontweight='bold', color=color, va='top')
ax.text(x+3.5, y+h-19.5, kpi_label, fontsize=10, color=SUB, va='top')
ax.text(x+3.5, y+h-23, body, fontsize=10.5, color=INK, va='top', linespacing=1.6)
ax.add_patch(patches.FancyBboxPatch((4, 4), 92, 6,
boxstyle="round,pad=0.3,rounding_size=1",
linewidth=0, facecolor=INK))
ax.text(50, 7, '一句话结论', ha='center', va='center',
fontsize=13, fontweight='bold', color='white')
plt.tight_layout()
plt.savefig('insights.png', dpi=150, bbox_inches='tight', facecolor='white')
图表 + 洞察卡 拼接
数据图和洞察卡画好后,用 PIL 拼接成完整版(适合长图分享):
from PIL import Image
img1 = Image.open('charts.png')
img2 = Image.open('insights.png')
target_w = max(img1.width, img2.width)
def pad(img, w):
if img.width == w: return img
new = Image.new('RGB', (w, img.height), (255,255,255))
new.paste(img, ((w - img.width)//2, 0))
return new
img1, img2 = pad(img1, target_w), pad(img2, target_w)
gap = 30
combined = Image.new('RGB', (target_w, img1.height + img2.height + gap), (255,255,255))
combined.paste(img1, (0, 0))
combined.paste(img2, (0, img1.height + gap))
combined.save('combined.png', dpi=(150,150), optimize=True)
横向拼接同理(统一高度,宽度相加),适合宽屏展示。