Specifying the line width of the legend frame in Matplotlib

To specify the line width of the legend frame in Matplotlib, we can use the set_linewidth() method. This allows you to customize the appearance of legend lines independently from the actual plot lines.

Basic Example

Here's how to modify the line width of legend entries ?

import numpy as np
import matplotlib.pyplot as plt

# Set figure size
plt.rcParams["figure.figsize"] = [7.50, 3.50]
plt.rcParams["figure.autolayout"] = True

# Create data points
x = np.linspace(-5, 5, 100)
y = np.sin(x)

# Create plot
fig, ax = plt.subplots()
ax.plot(x, y, c='r', label='y=sin(x)', linewidth=3.0)

# Create legend and modify line width
leg = plt.legend()
leg.get_lines()[0].set_linewidth(6)

plt.show()

Multiple Legend Lines

When working with multiple lines, you can set different widths for each legend entry ?

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(-5, 5, 100)
y1 = np.sin(x)
y2 = np.cos(x)

fig, ax = plt.subplots(figsize=(8, 4))
ax.plot(x, y1, 'r-', label='sin(x)', linewidth=2)
ax.plot(x, y2, 'b--', label='cos(x)', linewidth=2)

# Get legend and modify line widths
leg = plt.legend()
leg.get_lines()[0].set_linewidth(8)  # sin(x) legend line
leg.get_lines()[1].set_linewidth(4)  # cos(x) legend line

plt.grid(True, alpha=0.3)
plt.show()

Alternative Method Using Legend Properties

You can also set legend line properties during legend creation ?

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 10, 50)
y1 = np.exp(-x/5) * np.cos(x)
y2 = np.exp(-x/5) * np.sin(x)

plt.figure(figsize=(8, 5))
line1, = plt.plot(x, y1, 'g-', label='Damped Cosine', linewidth=1.5)
line2, = plt.plot(x, y2, 'm-', label='Damped Sine', linewidth=1.5)

# Create legend with custom properties
leg = plt.legend()
for line, width in zip(leg.get_lines(), [5, 3]):
    line.set_linewidth(width)

plt.xlabel('x')
plt.ylabel('y')
plt.title('Legend Line Width Customization')
plt.grid(True, alpha=0.3)
plt.show()

Key Points

• plt.legend() returns a legend object that contains line properties
• get_lines()[index] accesses individual legend lines by index
• set_linewidth() modifies only the legend line appearance, not the plot line
• Legend line order matches the order of plot creation

Conclusion

Use leg.get_lines()[index].set_linewidth(width) to customize legend line widths in Matplotlib. This technique helps emphasize important data series in your legend without affecting the actual plot lines.