Custom legend entries in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What does this code output?

Consider the following matplotlib code that creates a plot and a custom legend entry. What will be the number of legend entries shown?

Matplotlib

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

plt.plot([1, 2, 3], [4, 5, 6], label='Line 1')
custom_patch = mpatches.Patch(color='red', label='Custom Patch')
plt.legend(handles=[custom_patch])
plt.close()

A 3

B 1

C 0

D 2

💡 Hint

The legend uses only the handles provided explicitly.

The legend is created only with the custom_patch handle, so it shows exactly one legend entry labeled 'Custom Patch'. The original plot line is not included because it is not passed to legend.

❓ data_output

intermediate

What labels appear in the legend?

Given this code snippet, which labels will appear in the legend?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.lines as mlines

line1, = plt.plot([1, 2, 3], [1, 4, 9], label='Squares')
line2, = plt.plot([1, 2, 3], [1, 8, 27], label='Cubes')
custom_line = mlines.Line2D([], [], color='green', marker='o', linestyle='None', label='Custom Point')
plt.legend(handles=[line1, custom_line])
plt.close()

A Squares, Custom Point

B Squares, Cubes, Custom Point

C Cubes, Custom Point

D Squares, Cubes

💡 Hint

Only the handles passed to legend are shown.

The legend is created with handles=[line1, custom_line], so only 'Squares' and 'Custom Point' labels appear.

❓ visualization

advanced

Identify the correct legend appearance

Which option shows a legend with two entries: a blue line labeled 'Data' and a red square labeled 'Threshold'?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.patches as mpatches
import matplotlib.lines as mlines

plt.plot([0, 1, 2], [2, 3, 4], color='blue', label='Data')
red_square = mpatches.Patch(color='red', label='Threshold')
plt.legend(handles=[mlines.Line2D([], [], color='blue', label='Data'), red_square])
plt.close()

A Legend with a blue line labeled 'Data' only

B Legend with a red square labeled 'Threshold' only

C Legend with a blue line labeled 'Data' and a red square labeled 'Threshold'

D Legend with no entries

💡 Hint

Custom legend entries can combine Line2D and Patch objects.

The legend uses a blue Line2D object and a red Patch, so both appear with correct labels.

🔧 Debug

advanced

Why does this custom legend not show the label?

This code tries to add a custom legend entry but the label does not appear. What is the cause?

Matplotlib

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

plt.plot([1, 2, 3], [3, 2, 1])
custom_patch = mpatches.Patch(color='orange')
plt.legend(handles=[custom_patch])
plt.close()

A Legend requires plt.show() to display labels

B The plot line must be included in handles to show any legend

C The color 'orange' is invalid and causes the label to hide

D The Patch object lacks a label argument, so no label appears

💡 Hint

Check if the legend entries have labels.

The Patch was created without a label, so the legend entry is empty and no label is shown.

🚀 Application

expert

Create a custom legend with mixed marker and line styles

You want a legend with three entries: a green dashed line labeled 'Trend', a red circle marker labeled 'Data Point', and a blue square patch labeled 'Region'. Which code snippet correctly creates this custom legend?

A

import matplotlib.pyplot as plt
import matplotlib.lines as mlines
import matplotlib.patches as mpatches

trend_line = mlines.Line2D([], [], color='green', linestyle='--', label='Trend')
data_point = mlines.Line2D([], [], color='red', marker='o', linestyle='None', label='Data Point')
region_patch = mpatches.Patch(color='blue', label='Region')
plt.legend(handles=[trend_line, data_point, region_patch])

B

import matplotlib.pyplot as plt
import matplotlib.lines as mlines
import matplotlib.patches as mpatches

trend_line = mlines.Line2D([], [], color='green', linestyle='-', label='Trend')
data_point = mpatches.Circle((0,0), radius=5, color='red', label='Data Point')
region_patch = mpatches.Rectangle((0,0), 1, 1, color='blue', label='Region')
plt.legend(handles=[trend_line, data_point, region_patch])

C

import matplotlib.pyplot as plt
import matplotlib.lines as mlines
import matplotlib.patches as mpatches

trend_line = mlines.Line2D([], [], color='green', linestyle='--', label='Trend')
data_point = mlines.Line2D([], [], color='red', marker='o', label='Data Point')
region_patch = mpatches.Patch(color='blue', label='Region')
plt.legend(handles=[trend_line, data_point, region_patch])

D

import matplotlib.pyplot as plt
import matplotlib.lines as mlines
import matplotlib.patches as mpatches

trend_line = mlines.Line2D([], [], color='green', linestyle='--', label='Trend')
data_point = mlines.Line2D([], [], color='red', marker='o', linestyle='-', label='Data Point')
region_patch = mpatches.Patch(color='blue', label='Region')
plt.legend(handles=[trend_line, data_point, region_patch])

💡 Hint

Markers with no connecting line use linestyle='None'.

Option A correctly sets linestyle='--' for dashed line, linestyle='None' for marker only, and uses Patch for the blue square.