Line2D and Artists¶

Artist Base Class¶

1. Hierarchy¶

All plot elements inherit from Artist:

```python import matplotlib.pyplot as plt

fig, ax = plt.subplots() line, = ax.plot([1, 2, 3]) print(isinstance(line, plt.Artist)) # True ```

2. Properties¶

python line.set_color('red') line.set_linewidth(2) line.set_linestyle('--') line.set_marker('o') line.set_markersize(8)

3. Visibility¶

python line.set_visible(False) line.set_alpha(0.5) # Transparency

Line2D¶

1. Creation¶

```python from matplotlib.lines import Line2D

line = Line2D([0, 1, 2], [0, 1, 0], color='blue', linewidth=2) ax.add_line(line) ```

2. Properties¶

python line.set_xdata([0, 1, 2, 3]) line.set_ydata([0, 1, 0, 1]) line.set_label('Custom Line')

3. Markers¶

python line.set_marker('o') line.set_markerfacecolor('yellow') line.set_markeredgecolor('black') line.set_markeredgewidth(1)

Other Artists¶

1. Text¶

python text = ax.text(0.5, 0.5, 'Hello', fontsize=14) text.set_color('red') text.set_rotation(45)

2. Patch¶

```python from matplotlib.patches import Rectangle

rect = Rectangle((0, 0), 1, 1, facecolor='lightblue', edgecolor='blue') ax.add_patch(rect) ```

3. Collection¶

```python from matplotlib.collections import LineCollection

segments = [[(0, 0), (1, 1)], [(1, 0), (0, 1)]] lc = LineCollection(segments, colors='red', linewidths=2) ax.add_collection(lc) ```

The One Abstraction¶

Every Artist is a stateful object with properties and a draw() method. That's it. Lines, text, patches, ticks, legends --- they all follow the same pattern:

python artist.set_color('red') # change a property artist.get_linewidth() # read a property artist.set_visible(False) # hide it

Debugging with Artists¶

When a plot looks wrong, inspect the Artist tree:

```python fig, ax = plt.subplots() ax.plot([1, 2, 3], [1, 4, 9])

List all artists on the axes¶

for child in ax.get_children(): print(type(child).name, getattr(child, 'get_label', lambda: '')())

Inspect a specific line¶

lines = ax.get_lines() for line in lines: print(f"color={line.get_color()}, lw={line.get_linewidth()}, visible={line.get_visible()}") ```

Common issues:

• Plot not showing? → Check artist.get_visible() and axis limits
• Wrong z-order? → Check artist.get_zorder() --- higher values draw on top
• Plot not updating? → Call fig.canvas.draw() after modifying Artists

Exercises¶

Exercise 1. Write code that creates a plot and uses ax.get_children() to list all Artist objects. Print the type of each child.

Exercise 2. Explain the Matplotlib Artist hierarchy. What is the base class for all drawable objects in Matplotlib?

Exercise 3. Write code that creates a Line2D object with ax.plot(), then modifies its properties using setter methods (e.g., line.set_color()).

Exercise 4. Create a plot and manually add a matplotlib.patches.Rectangle to the axes using ax.add_patch().

import matplotlib.pyplot as plt
import matplotlib.patches as mpatches
import numpy as np

fig, ax = plt.subplots()
x = np.linspace(0, 10, 100)
ax.plot(x, np.sin(x), 'b-')

# Manually create and add a Rectangle Artist
rect = mpatches.Rectangle((2, -0.5), 3, 1,
                           linewidth=2, edgecolor='red',
                           facecolor='yellow', alpha=0.3)
ax.add_patch(rect)

ax.set_title('Line2D + manually added Rectangle')
plt.show()

# The Rectangle is a Patch Artist — same base class as bars in bar charts.
# add_patch() inserts it into the Axes' artist list for rendering.

Exercise 5. Demonstrate the set_*() / get_*() pattern. Create a line plot and then modify it after creation: change the line color, width, linestyle, and marker using only set_*() methods on the returned Line2D object. Print the properties before and after using get_color(), get_linewidth(), etc.

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()
x = np.linspace(0, 5, 30)
line, = ax.plot(x, np.cos(x))

# Before modification
print(f"Color:     {line.get_color()}")
print(f"Linewidth: {line.get_linewidth()}")
print(f"Linestyle: {line.get_linestyle()}")

# Modify via set_*() — no need to replot
line.set_color('red')
line.set_linewidth(3)
line.set_linestyle('--')
line.set_marker('o')
line.set_markersize(6)

# After modification
print(f"\nAfter set_*():")
print(f"Color:     {line.get_color()}")
print(f"Linewidth: {line.get_linewidth()}")
print(f"Linestyle: {line.get_linestyle()}")

ax.set_title('Modified Line2D (red, dashed, markers)')
plt.show()

# This is the Artist mutation model: create once, modify freely.
# The plot updates when drawn — no need to recreate the figure.