Annotating specific points in Matplotlib - Step-by-Step Execution

Concept Flow - Annotating specific points

Plot data points

↓

Choose point to annotate

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Add annotation text

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Set arrow properties

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Display plot with annotation

The flow shows plotting data, selecting a point, adding annotation text with an arrow, and displaying the plot.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [2, 3, 5]
plt.scatter(x, y)
plt.annotate('Max', xy=(3, 5), xytext=(2, 4), arrowprops=dict(facecolor='black'))
plt.show()

This code plots points and annotates the point (3,5) with the label 'Max' and an arrow.

Execution Table

Step	Action	Input/Parameters	Effect on Plot	Annotation State
1	Plot scatter points	x=[1,2,3], y=[2,3,5]	Points appear at (1,2), (2,3), (3,5)	No annotations yet
2	Call annotate()	text='Max', xy=(3,5), xytext=(2,4), arrowprops=facecolor black	Arrow from (2,4) to (3,5) added	Annotation text 'Max' placed at (2,4)
3	Display plot	plt.show()	Plot window opens showing points and annotation	Annotation visible with arrow pointing to (3,5)

💡 Plot displayed with points and annotation; execution ends.

Variable Tracker

Variable	Start	After Plot	After Annotate	Final
x	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]
y	[2, 3, 5]	[2, 3, 5]	[2, 3, 5]	[2, 3, 5]
annotation_text	None	None	'Max'	'Max'
annotation_position_xy	None	None	(3, 5)	(3, 5)
annotation_position_xytext	None	None	(2, 4)	(2, 4)
arrow_properties	None	None	{'facecolor': 'black'}	{'facecolor': 'black'}

Key Moments - 3 Insights

Why does the annotation text appear at (2,4) but the arrow points to (3,5)?

The annotate() function uses xytext for the text position and xy for the point being annotated. The arrow connects xytext to xy, as shown in execution_table step 2.

What happens if arrowprops is omitted in annotate()?

Without arrowprops, no arrow is drawn. The text appears at xytext but no visual arrow points to xy. This is implied by the arrowprops effect in execution_table step 2.

Can we annotate a point not in the data?

Yes, annotate() can point anywhere on the plot. The xy parameter sets the arrow tip location, which can be outside the data points, as long as it is within plot limits.

Visual Quiz - 3 Questions

Look at the execution_table at step 2. Where is the annotation text placed?

A At (2, 4)

B At (3, 5)

C At (1, 2)

D At (0, 0)

💡 Hint

Check the 'Annotation State' column in step 2 of execution_table.

According to variable_tracker, what is the value of arrow_properties after annotate()?

A None

B {'color': 'red'}

C {'facecolor': 'black'}

D {}

💡 Hint

Look at the 'arrow_properties' row after 'After Annotate' in variable_tracker.

If we remove arrowprops from annotate(), what changes in the plot?

A The annotation text disappears

B The arrow is not drawn

C The points disappear

D The plot crashes

💡 Hint

Refer to key_moments explanation about arrowprops effect.

Concept Snapshot

matplotlib annotate() adds text labels with optional arrows.
Use xy for point location, xytext for text position.
arrowprops controls arrow style.
Useful to highlight specific data points on plots.

Full Transcript

This visual execution traces how matplotlib's annotate() function works to label specific points on a plot. First, data points are plotted using scatter(). Then annotate() is called with text, the point to annotate (xy), the text position (xytext), and arrow properties. The arrow connects the text to the point. Finally, plt.show() displays the plot with points and annotation. Variables like annotation_text, annotation_position_xy, and arrow_properties update accordingly. Key moments clarify why text and arrow positions differ and the role of arrowprops. The quiz tests understanding of annotation placement and arrow behavior.