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Labels and Legends

Labels and legends help viewers understand what data is being displayed.

Adding Labels

Use the label parameter in plot():

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-2*np.pi, 2*np.pi, 100)
y_sin = np.sin(x)
y_cos = np.cos(x)

fig, ax = plt.subplots(figsize=(15, 4))
ax.plot(x, y_sin, label='sin')
ax.plot(x, y_cos, label='cos')
ax.legend()
plt.show()

Alternative: Labels in legend()

Pass labels directly to legend():

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-6, 6, 100)
y_sin = np.sin(x)
y_cos = np.cos(x)

fig, ax = plt.subplots(figsize=(12, 3))
ax.plot(x, y_sin)
ax.plot(x, y_cos)
ax.legend(["sin", "cos"])
plt.show()

Legend Location (loc)

Control legend placement:

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-6, 6, 100)

fig, ax = plt.subplots(figsize=(12, 3))
ax.plot(x, np.sin(x), label="sin")
ax.plot(x, np.cos(x), label="cos")
ax.legend(loc='upper right')
plt.show()

Location options:

Value Description
'best' Auto-select best location (default)
'upper right' Top right
'upper left' Top left
'lower left' Bottom left
'lower right' Bottom right
'right' Right side
'center left' Left center
'center right' Right center
'lower center' Bottom center
'upper center' Top center
'center' Center

Numeric codes (0-10) also work.

Legend Font Size

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-6, 6, 100)

fig, ax = plt.subplots(figsize=(12, 3))
ax.plot(x, np.sin(x), label="sin")
ax.plot(x, np.cos(x), label="cos")
ax.legend(fontsize=20)
plt.show()

Legend Shadow

Add a shadow effect:

ax.legend(shadow=True)

Legend Frame

Remove the frame border:

ax.legend(frameon=False)

Multiple Columns (ncol)

Arrange legend entries in columns:

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-1, 1, 100)

fig, ax = plt.subplots(figsize=(12, 3))

for beta in range(1, 9):
    ax.plot(x, beta * x, label=f"y(x)={beta}*x")

ax.legend(ncol=4)
plt.show()

Legend Outside Plot (bbox_to_anchor)

Place legend outside the axes:

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-1, 1, 100)

fig, ax = plt.subplots(figsize=(12, 3))

for beta in range(1, 9):
    ax.plot(x, beta * x, label=f"y(x)={beta}*x")

ax.legend(ncol=4, loc='lower left', bbox_to_anchor=(0, 1))
plt.show()

The bbox_to_anchor parameter takes (x, y) coordinates in axes fraction.

Comprehensive Legend Example

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(-2*np.pi, 2*np.pi, 100)

fig, ax = plt.subplots(figsize=(15, 4))

for beta in range(1, 9):
    ax.plot(x, beta * x, label=f"y(x)={beta}*x")

ax.legend(
    fontsize=15,
    shadow=True,
    ncol=4,
    loc='lower right'
)
plt.show()

Legend Title

Add a title to the legend:

ax.legend(title='Functions', title_fontsize=14)

Selective Legend Entries

Only include specific lines in the legend:

import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(0, 10, 100)

fig, ax = plt.subplots()

line1, = ax.plot(x, np.sin(x), label='sin')
line2, = ax.plot(x, np.cos(x), label='cos')
ax.plot(x, x/10)  # No label, won't appear in legend

ax.legend()  # Only shows sin and cos
plt.show()

Key Takeaways

  • Add label parameter to plot() calls
  • Call ax.legend() to display the legend
  • Use loc to position the legend
  • Use ncol for multi-column layouts
  • Use bbox_to_anchor for legends outside the plot
  • frameon=False removes the border
  • shadow=True adds a drop shadow