/matplotlib

name: matplotlib
description: "Foundational plotting library. Create line plots, scatter, bar, histograms, heatmaps, 3D, subplots, export PNG/PDF/SVG, for scientific visualization and publication figures."

Matplotlib

Overview

Matplotlib is Python's foundational visualization library for creating static, animated, and interactive plots. This skill provides guidance on using matplotlib effectively, covering both the pyplot interface (MATLAB-style) and the object-oriented API (Figure/Axes), along with best practices for creating publication-quality visualizations.

When to Use This Skill

This skill should be used when:

• Creating any type of plot or chart (line, scatter, bar, histogram, heatmap, contour, etc.)
• Generating scientific or statistical visualizations
• Customizing plot appearance (colors, styles, labels, legends)
• Creating multi-panel figures with subplots
• Exporting visualizations to various formats (PNG, PDF, SVG, etc.)
• Building interactive plots or animations
• Working with 3D visualizations
• Integrating plots into Jupyter notebooks or GUI applications

Core Concepts

The Matplotlib Hierarchy

Matplotlib uses a hierarchical structure of objects:

1. **Figure** - The top-level container for all plot elements 2. **Axes** - The actual plotting area where data is displayed (one Figure can contain multiple Axes) 3. **Artist** - Everything visible on the figure (lines, text, ticks, etc.) 4. **Axis** - The number line objects (x-axis, y-axis) that handle ticks and labels

Two Interfaces

**1. pyplot Interface (Implicit, MATLAB-style)**

import matplotlib.pyplot as plt

plt.plot([1, 2, 3, 4])
plt.ylabel('some numbers')
plt.show()

• Convenient for quick, simple plots
• Maintains state automatically
• Good for interactive work and simple scripts

**2. Object-Oriented Interface (Explicit)**

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.plot([1, 2, 3, 4])
ax.set_ylabel('some numbers')
plt.show()

• **Recommended for most use cases**
• More explicit control over figure and axes
• Better for complex figures with multiple subplots
• Easier to maintain and debug

Common Workflows

1. Basic Plot Creation

**Single plot workflow:**

import matplotlib.pyplot as plt
import numpy as np

# Create figure and axes (OO interface - RECOMMENDED)
fig, ax = plt.subplots(figsize=(10, 6))

# Generate and plot data
x = np.linspace(0, 2*np.pi, 100)
ax.plot(x, np.sin(x), label='sin(x)')
ax.plot(x, np.cos(x), label='cos(x)')

# Customize
ax.set_xlabel('x')
ax.set_ylabel('y')
ax.set_title('Trigonometric Functions')
ax.legend()
ax.grid(True, alpha=0.3)

# Save and/or display
plt.savefig('plot.png', dpi=300, bbox_inches='tight')
plt.show()

2. Multiple Subplots

**Creating subplot layouts:**

# Method 1: Regular grid
fig, axes = plt.subplots(2, 2, figsize=(12, 10))
axes[0, 0].plot(x, y1)
axes[0, 1].scatter(x, y2)
axes[1, 0].bar(categories, values)
axes[1, 1].hist(data, bins=30)

# Method 2: Mosaic layout (more flexible)
fig, axes = plt.subplot_mosaic([['left', 'right_top'],
                                 ['left', 'right_bottom']],
                                figsize=(10, 8))
axes['left'].plot(x, y)
axes['right_top'].scatter(x, y)
axes['right_bottom'].hist(data)

# Method 3: GridSpec (maximum control)
from matplotlib.gridspec import GridSpec
fig = plt.figure(figsize=(12, 8))
gs = GridSpec(3, 3, figure=fig)
ax1 = fig.add_subplot(gs[0, :])  # Top row, all columns
ax2 = fig.add_subplot(gs[1:, 0])  # Bottom two rows, first column
ax3 = fig.add_subplot(gs[1:, 1:])  # Bottom two rows, last two columns

3. Plot Types and Use Cases

**Line plots** - Time series, continuous data, trends

ax.plot(x, y, linewidth=2, linestyle='--', marker='o', color='blue')

**Scatter plots** - Relationships between variables, correlations

ax.scatter(x, y, s=sizes, c=colors, alpha=0.6, cmap='viridis')

**Bar charts** - Categorical comparisons

ax.bar(categories, values, color='steelblue', edgecolor='black')
# For horizontal bars:
ax.barh(categories, values)

**Histograms** - Distributions

ax.hist(data, bins=30, edgecolor='black', alpha=0.7)

**Heatmaps** - Matrix data, correlations

im = ax.imshow(matrix, cmap='coolwarm', aspect='auto')
plt.colorbar(im, ax=ax)

**Contour plots** - 3D data on 2D plane

contour = ax.contour(X, Y, Z, levels=10)
ax.clabel(contour, inline=True, fontsize=8)

**Box plots** - Statistical distributions

ax.boxplot([data1, data2, data3], labels=['A', 'B', 'C'])

**Violin plots** - Distribution densities

ax.violinplot([data1, data2, data3], positions=[1, 2, 3])

For comprehensive plot type examples and variations, refer to `references/plot_types.md`.

4. Styling and Customization

**Color specification methods:**

• Named colors: `'red'`, `'blue'`, `'steelblue'`
• Hex codes: `'#FF5733'`
• RGB tuples: `(0.1, 0.2, 0.3)`
• Colormaps: `cmap='viridis'`, `cmap='plasma'`, `cmap='coolwarm'`

**Using style sheets:**

plt.style.use('seaborn-v0_8-darkgrid')  # Apply predefined style
# Available styles: 'ggplot', 'bmh', 'fivethirtyeight', etc.
print(plt.style.available)  # List all available styles

**Customizing with rcParams:**

plt.rcParams['font.size'] = 12
plt.rcParams['axes.labelsize'] = 14
plt.rcParams['axes.titlesize'] = 16
plt.rcParams['xtick.labelsize'] = 10
plt.rcParams['ytick.labelsize'] = 10
plt.rcParams['legend.fontsize'] = 12
plt.rcParams['figure.titlesize'] = 18

**Text and annotations:**

ax.text(x, y, 'annotation', fontsize=12, ha='center')
ax.annotate('important point', xy=(x, y), xytext=(x+1, y+1),
            arrowprops=dict(arrowstyle='->', color='red'))

For detailed styling options and colormap guidelines, see `references/styling_guide.md`.

5. Saving Figures

**Export to various formats:**

# High-resolution PNG for presenta