Custom legend entries in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

Custom legend entries allow you to add specific labels and symbols to the legend that may not directly correspond to plotted data, helping to explain or highlight important information clearly.

beginner

Which matplotlib class is commonly used to create custom legend handles?

The <code>matplotlib.patches.Patch</code> class is often used to create custom legend handles with specific colors and labels.

intermediate

How do you add a custom legend entry without plotting a new line or marker?

You create a custom handle (like a Patch or Line2D object) and pass it along with a label to the legend() function using the handles and labels parameters.

intermediate

What is the role of Line2D in custom legend entries?

Line2D can be used to create custom line or marker styles for legend entries, especially when you want to show a line or marker that is not part of the plotted data.

beginner

True or False: Custom legend entries can only represent colors, not shapes or line styles.

False. Custom legend entries can represent colors, shapes, line styles, and markers by using appropriate matplotlib objects like Patch or Line2D.

Which parameter do you use to specify custom legend handles in matplotlib?

A markers

B labels

C colors

D handles

The handles parameter is used to pass custom legend entries (like patches or lines) to the legend.

What matplotlib object would you use to create a colored box for a custom legend entry?

A Patch

B Line2D

C Figure

D Axes

Patch objects create colored shapes like boxes or rectangles for legend entries.

If you want to add a custom legend entry showing a dashed line, which object is best to use?

A Patch

B Text

C Line2D

D Scatter

Line2D can create custom line styles such as dashed lines for legend entries.

Can you add a legend entry that does not correspond to any plotted data?

A No, legend entries must match plotted data

B Yes, by creating custom handles

C Only if you plot invisible data

D Only with special matplotlib plugins

You can add custom legend entries by creating handles manually without plotting data.

Which function is used to display the legend in matplotlib?

A plt.legend()

B plt.plot()

C plt.show()

D plt.title()

plt.legend() displays the legend on the plot.

Explain how to create and add a custom legend entry in matplotlib without plotting new data.

Think about creating a shape or line object and passing it to legend.

Describe the difference between using Patch and Line2D for custom legend entries.

Consider what visual element you want to show in the legend.

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