Custom legend entries in Matplotlib - Step-by-Step Execution

Concept Flow - Custom legend entries

Create plot elements

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Define custom legend handles

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Call legend() with custom handles and labels

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Display plot with custom legend

First, create plot elements. Then define custom legend handles. Next, call legend() with these handles and labels. Finally, show the plot with the custom legend.

Execution Sample

Matplotlib

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

plt.plot([1,2,3], label='Line 1')
custom_handle = Line2D([0], [0], color='red', lw=2)
plt.legend(handles=[custom_handle], labels=['Custom Red Line'])
plt.show()

This code plots a line but shows a custom legend entry with a red line instead of the default.

Execution Table

Step	Action	Plot Elements	Legend Handles	Legend Labels	Result
1	Import matplotlib and Line2D	No plot yet	None	None	Ready to plot
2	Plot line with label 'Line 1'	Line plotted (default blue)	None	None	Line visible on plot
3	Create custom Line2D handle (red line)	Line plotted	Red line handle created	None	Custom handle ready
4	Call plt.legend() with custom handle and label	Line plotted	Red line handle	['Custom Red Line']	Legend shows red line with label
5	Show plot	Line plotted	Red line handle	['Custom Red Line']	Plot displayed with custom legend

💡 Plot displayed with custom legend replacing default legend entries

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
plt	matplotlib.pyplot module	plot created	plot created	plot created	plot displayed
custom_handle	undefined	undefined	Line2D object (red line)	Line2D object (red line)	Line2D object (red line)
legend handles	None	None	None	[custom_handle]	[custom_handle]
legend labels	None	None	None	['Custom Red Line']	['Custom Red Line']

Key Moments - 3 Insights

Why does the legend show the custom red line instead of the plotted blue line?

Because in step 4, plt.legend() is called with the custom handle and label, overriding the default legend entries from the plot (see execution_table row 4).

Can we add multiple custom legend entries not linked to plotted data?

Yes, by creating multiple custom handles and passing them all in the handles list with corresponding labels in plt.legend(), as shown in step 4.

What happens if we call plt.legend() without handles or labels?

Matplotlib automatically uses the labels from plotted elements. Custom handles override this behavior (see step 4 vs step 2).

Visual Quiz - 3 Questions

Look at the execution table at step 3, what is the state of 'custom_handle'?

A It is undefined

B It is a Line2D object representing a red line

C It is the plotted blue line

D It is a legend label string

💡 Hint

Check the 'custom_handle' column in variable_tracker after step 3

At which step does the legend start showing the custom red line instead of the default plot line?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Legend Handles' and 'Result' columns in execution_table

If we remove the 'handles' argument in plt.legend(), what will the legend show?

A No legend will appear

B The custom red line legend entry

C The default legend from plotted lines

D An error will occur

💡 Hint

Refer to key_moments about default legend behavior when no handles are passed

Concept Snapshot

Custom legend entries in matplotlib:
- Create plot elements normally
- Define custom legend handles (e.g., Line2D objects)
- Call plt.legend(handles=[...], labels=[...]) to override default legend
- Useful to show legend entries not directly linked to plotted data
- plt.show() displays the plot with custom legend

Full Transcript

This visual execution shows how to create custom legend entries in matplotlib. First, we import matplotlib and plot a line. Then, we create a custom legend handle using Line2D with a red color. Next, we call plt.legend() passing this custom handle and a label, which replaces the default legend entry. Finally, we display the plot with the custom legend. The variable tracker shows how the custom handle and legend labels are set step-by-step. Key moments clarify why the custom legend overrides the default and how to add multiple custom entries. The quiz tests understanding of when and how the custom legend appears.