How to make several legend keys to the same entry in Matplotlib?

In Matplotlib, you can create a legend entry with multiple keys (symbols) representing different lines or data series. This is useful when you want to group related plots under a single legend label.

Basic Approach

Use the HandlerTuple class to group multiple plot objects under one legend entry ?

import matplotlib.pyplot as plt
from matplotlib.legend_handler import HandlerTuple

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

# Create two different line plots
p1, = plt.plot([1, 2.5, 3], 'r-d', label='Red line')
p2, = plt.plot([3, 2, 1], 'k-o', label='Black line')

# Create legend with both keys for single entry
legend = plt.legend([(p1, p2)], ['Two keys'], numpoints=1, 
                   handler_map={tuple: HandlerTuple(ndivide=None)})

plt.show()

Multiple Legend Entries with Multiple Keys

You can create several legend entries, each with multiple keys ?

import matplotlib.pyplot as plt
from matplotlib.legend_handler import HandlerTuple

# Create multiple plots
line1, = plt.plot([1, 2, 3], 'r-o', markersize=8)
line2, = plt.plot([1, 2, 3], 'r--', linewidth=2)
line3, = plt.plot([3, 2, 1], 'b-s', markersize=8)
line4, = plt.plot([3, 2, 1], 'b:', linewidth=2)

# Create legend with multiple entries, each having multiple keys
plt.legend([(line1, line2), (line3, line4)], 
          ['Red Group', 'Blue Group'],
          handler_map={tuple: HandlerTuple(ndivide=None)})

plt.xlabel('X-axis')
plt.ylabel('Y-axis')
plt.title('Legend with Multiple Keys per Entry')
plt.show()

Customizing Handler Spacing

Control the spacing between keys using the ndivide parameter ?

import matplotlib.pyplot as plt
from matplotlib.legend_handler import HandlerTuple

p1, = plt.plot([1, 2.5, 3], 'r-d', markersize=10)
p2, = plt.plot([3, 2, 1], 'g-o', markersize=10)
p3, = plt.plot([2, 1.5, 2.5], 'b-^', markersize=10)

# Different spacing options
plt.legend([(p1, p2, p3)], ['Three keys with custom spacing'], 
          handler_map={tuple: HandlerTuple(ndivide=2)},
          loc='upper right')

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

Parameters

Conclusion

Use HandlerTuple with tuples of plot objects to create legend entries with multiple keys. This technique is perfect for grouping related data series under single legend labels.