Custom legend entries in Matplotlib - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the module needed for plotting.

Matplotlib

import [1] as plt

Drag options to blanks, or click blank then click option'

Aseaborn

Bmatplotlib.pyplot

Cnumpy

Dpandas

Attempts:

3 left

💡 Hint

Common Mistakes

Importing numpy or pandas instead of matplotlib.pyplot

Using seaborn which is a different plotting library

We import matplotlib.pyplot as plt to create plots and legends.

2fill in blank

medium

Complete the code to create a simple line plot with label 'Line 1'.

Matplotlib

plt.plot([1, 2, 3], [4, 5, 6], label=[1])
plt.legend()
plt.show()

Drag options to blanks, or click blank then click option'

Aline1

BLine 1

C'Line 1'

Dlabel

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes around the label text

Using a variable name without defining it

The label must be a string, so it needs quotes around it.

3fill in blank

hard

Fix the error in the code to create a custom legend entry using Line2D.

Matplotlib

from matplotlib.lines import Line2D
custom_line = Line2D([0], [0], color='red', lw=4)
plt.legend(handles=[[1]])
plt.show()

Drag options to blanks, or click blank then click option'

Acustom_line

Bline

Ccustom_lines

Dhandle

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name that was not defined

Using plural form when the variable is singular

The variable created is custom_line, so it must be used in the legend handles list.

4fill in blank

hard

Fill both blanks to create two custom legend entries with different colors and labels.

Matplotlib

from matplotlib.lines import Line2D
custom_lines = [Line2D([0], [0], color=[1], lw=4), Line2D([0], [0], color=[2], lw=4)]
plt.legend(custom_lines, ['Blue line', 'Green line'])
plt.show()

Drag options to blanks, or click blank then click option'

A'blue'

B'red'

C'green'

D'yellow'

Attempts:

3 left

💡 Hint

Common Mistakes

Using colors without quotes

Mixing up the order of colors and labels

The first custom line is blue and the second is green, so use 'blue' and 'green' strings.

5fill in blank

hard

Fill all three blanks to create a custom legend with shapes and labels.

Matplotlib

from matplotlib.lines import Line2D
custom_lines = [Line2D([0], [0], color='black', marker=[1], linestyle='None', markersize=10),
                Line2D([0], [0], color='black', marker=[2], linestyle='None', markersize=10),
                Line2D([0], [0], color='black', marker=[3], linestyle='None', markersize=10)]
plt.legend(custom_lines, ['Circle', 'Square', 'Triangle'])
plt.show()

Drag options to blanks, or click blank then click option'

A'o'

B's'

C'^'

D'x'

Attempts:

3 left

💡 Hint

Common Mistakes

Using marker characters that do not match the shape labels

Forgetting quotes around marker characters

The markers 'o', 's', and '^' represent circle, square, and triangle shapes respectively.

Practice

1. What is the main purpose of creating custom legend entries in matplotlib?

easy

A. To change the plot background color

B. To increase the plot size

C. To add labels for plot elements that are not automatically labeled

D. To save the plot as an image file

Solution

1. Step 1: Understand legend purpose

   Legends explain what different plot elements represent.

2. Step 2: Identify custom legend role

   Custom legend entries add labels for elements not labeled automatically.

3. Final Answer:

   To add labels for plot elements that are not automatically labeled -> Option C

4. Quick Check:

   Custom legend entries = add labels [OK]

Hint: Custom legends label unlabeled plot parts quickly [OK]

Common Mistakes:

• Confusing legend with plot size or color
• Thinking legend changes plot background
• Assuming legend saves the plot

2. Which of the following is the correct way to create a custom legend entry for a red line using matplotlib.lines.Line2D?

easy

A. Line2D([0], [0], color='red', label='Red Line')

B. Line2D(color='red', label='Red Line')

C. Line2D([0,1], [0,1], color='red')

D. Line2D([0], [0], 'red', label='Red Line')

Solution

1. Step 1: Check Line2D constructor

   Line2D requires x and y data points as first two arguments.

2. Step 2: Verify correct syntax

   Line2D([0], [0], color='red', label='Red Line') uses [0], [0] for points, color='red', and label='Red Line' correctly.

3. Final Answer:

   Line2D([0], [0], color='red', label='Red Line') -> Option A

4. Quick Check:

   Line2D needs x,y points + color + label [OK]

Hint: Line2D needs x and y lists first, then style [OK]

Common Mistakes:

• Omitting x and y data points
• Passing color as positional argument
• Missing label parameter

3. What will be the output legend labels for this code snippet?

import matplotlib.pyplot as plt
from matplotlib.lines import Line2D

plt.plot([1, 2, 3], [4, 5, 6], 'b-')
custom_line = Line2D([0], [0], color='red', lw=2)
plt.legend([custom_line], ['Custom Red Line'])
plt.show()

medium

A. A legend with two labels: 'Custom Red Line' and 'blue line'

B. A legend with one label: 'blue line' shown in blue

C. No legend will appear

D. A legend with one label: 'Custom Red Line' shown in red

Solution

1. Step 1: Analyze plot and legend calls

   Plot draws a blue line but legend uses only custom_line handle with label 'Custom Red Line'.

2. Step 2: Understand legend content

   Legend shows only what is passed: one red line with label 'Custom Red Line'.

3. Final Answer:

   A legend with one label: 'Custom Red Line' shown in red -> Option D

4. Quick Check:

   Legend shows passed handles and labels only [OK]

Hint: Legend shows only handles and labels you pass [OK]

Common Mistakes:

• Assuming plot lines auto-appear in legend
• Expecting multiple legend entries without passing handles
• Thinking legend is empty

4. Identify the error in this code that tries to add a custom legend entry for a green square patch:

import matplotlib.pyplot as plt
from matplotlib.patches import Patch

patch = Patch(color='green', label='Green Square')
plt.legend(handles=patch, labels=['Green Square'])
plt.show()

medium

A. Patch cannot be used for legend entries

B. The 'handles' argument should be a list, not a single Patch object

C. The 'label' parameter is missing in Patch

D. plt.show() must come before plt.legend()

Solution

1. Step 1: Check handles argument type

   plt.legend expects handles as a list or iterable, not a single object.

2. Step 2: Identify fix

   Wrap patch in a list: handles=[patch] to fix the error.

3. Final Answer:

   The 'handles' argument should be a list, not a single Patch object -> Option B

4. Quick Check:

   Handles must be list or iterable [OK]

Hint: Always pass handles as a list to plt.legend [OK]

Common Mistakes:

• Passing single object instead of list to handles
• Ignoring label parameter in Patch
• Calling plt.show() before plt.legend()

5. You want to create a legend with a blue dashed line labeled 'Trend' and a red circle marker labeled 'Data Point'. Which code correctly creates custom legend entries for this?

hard

A. from matplotlib.lines import Line2D
legend_elements = [Line2D([0], [0], color='blue', lw=2, linestyle='--', label='Trend'), Line2D([0], [0], marker='o', color='red', linestyle='', label='Data Point')]
plt.legend(handles=legend_elements)

B. from matplotlib.patches import Patch
legend_elements = [Patch(color='blue', label='Trend'), Patch(color='red', label='Data Point')]
plt.legend(handles=legend_elements)

C. plt.legend(['Trend', 'Data Point'], loc='best')

D. plt.legend(['blue dashed line', 'red circle'])

Solution

1. Step 1: Identify correct legend handles

   Blue dashed line and red circle marker require Line2D objects with correct styles.

2. Step 2: Verify code correctness

   from matplotlib.lines import Line2D legend_elements = [Line2D([0], [0], color='blue', lw=2, linestyle='--', label='Trend'), Line2D([0], [0], marker='o', color='red', linestyle='', label='Data Point')] plt.legend(handles=legend_elements) creates Line2D with color='blue', linestyle='--' for 'Trend' and marker='o', color='red' for 'Data Point', then passes handles to plt.legend.

3. Final Answer:

   from matplotlib.lines import Line2D legend_elements = [Line2D([0], [0], color='blue', lw=2, linestyle='--', label='Trend'), Line2D([0], [0], marker='o', color='red', linestyle='', label='Data Point')] plt.legend(handles=legend_elements) -> Option A

4. Quick Check:

   Use Line2D with style and marker for custom legend [OK]

Hint: Use Line2D with marker and linestyle for custom legend [OK]

Common Mistakes:

• Using Patch for line and marker legend
• Passing only label strings without handles
• Ignoring linestyle or marker parameters