Text annotations in Matplotlib - Step-by-Step Execution

Concept Flow - Text annotations

Create plot data

↓

Plot data points

↓

Add annotation text

↓

Set annotation position

↓

Display plot with annotation

The flow shows creating data, plotting it, adding text annotations at specific points, and displaying the final plot.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [4, 5, 6]
plt.plot(x, y, 'o')
plt.annotate('Point 2', (2, 5))
plt.show()

This code plots three points and adds a text annotation 'Point 2' at the coordinates (2, 5).

Execution Table

Step	Action	Data/Variables	Result/Output
1	Import matplotlib.pyplot as plt	plt module imported	Ready to plot
2	Define x and y data	x=[1,2,3], y=[4,5,6]	Data ready for plotting
3	Plot points with 'o' marker	x and y lists	Scatter plot with 3 points displayed
4	Add annotation text 'Point 2' at (2,5)	Annotation text and position	Text label appears near point (2,5)
5	Show plot	Plot with points and annotation	Plot window opens showing points and annotation
6	End	N/A	Plot displayed, execution complete

💡 Plot displayed with points and annotation; program ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
x	undefined	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]	[1, 2, 3]
y	undefined	[4, 5, 6]	[4, 5, 6]	[4, 5, 6]	[4, 5, 6]
plt	undefined	matplotlib.pyplot module	plot object created	annotation added	plot shown

Key Moments - 2 Insights

Why does the annotation text appear at (2, 5) and not at the data point (2, 5) exactly?

The annotation position is set at (2, 5) but matplotlib places the text slightly offset by default to avoid overlapping the marker, as seen in step 4 of the execution_table.

What happens if we add annotation before plotting points?

If annotation is added before plotting, the text may appear but without points plotted yet, so the visual context is missing. The execution_table shows plotting happens first (step 3) before annotation (step 4).

Visual Quiz - 3 Questions

Look at the execution_table at step 4, what is the annotation text added?

A "Point 3"

B "Point 2"

C "Data Point"

D "Annotation"

💡 Hint

Check the 'Action' and 'Result/Output' columns at step 4 in execution_table.

At which step does the plot with points first appear?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look at the 'Result/Output' column to find when points are plotted.

If we change annotation position to (3, 6), how does the variable 'plt' state change after step 4?

A No change in annotation position

B Annotation removed

C Annotation added at (3, 6)

D Plot points removed

💡 Hint

Refer to variable_tracker and execution_table step 4 about annotation position.

Concept Snapshot

Text annotations in matplotlib:
- Use plt.annotate(text, (x, y)) to add labels
- Place text near data points for clarity
- Plot data first, then annotate
- Annotation position is coordinate-based
- plt.show() displays the final plot

Full Transcript

This lesson shows how to add text annotations to a matplotlib plot. First, we import matplotlib.pyplot as plt. Then we create data lists x and y. Next, we plot these points using plt.plot with circle markers. After plotting, we add a text annotation using plt.annotate with the label 'Point 2' at coordinates (2, 5). Finally, plt.show() displays the plot with points and the annotation. The annotation text appears near the specified point but slightly offset to avoid overlap. The execution table traces each step, and the variable tracker shows how variables change. Key moments clarify common confusions about annotation placement and order of plotting. The visual quiz tests understanding of annotation text, plotting steps, and position changes. The snapshot summarizes how to use text annotations effectively in matplotlib.

Practice

1. What is the main purpose of adding text annotations in a matplotlib plot?

easy

A. To add labels or notes that explain or highlight data points

B. To change the color of the plot lines

C. To increase the size of the plot window

D. To save the plot as an image file

Solution

1. Step 1: Understand what text annotations do

   Text annotations add extra information like labels or notes on specific parts of a plot.

2. Step 2: Identify the correct purpose

   Changing colors or saving files are unrelated to annotations, which focus on explaining or highlighting data points.

3. Final Answer:

   To add labels or notes that explain or highlight data points -> Option A

4. Quick Check:

   Annotations = labels/notes [OK]

Hint: Annotations explain data points clearly on plots [OK]

Common Mistakes:

• Confusing annotations with plot styling
• Thinking annotations save or export plots
• Assuming annotations change plot size

2. Which of the following is the correct syntax to add a text annotation at point (2, 3) with the text 'Peak' in matplotlib?

easy

A. plt.annotate(xy=(2, 3), text='Peak')

B. plt.text('Peak', xy=(2, 3))

C. plt.annotate('Peak', xy=(2, 3))

D. plt.text(x=2, y=3, label='Peak')

Solution

1. Step 1: Recall the correct function and parameter order

   The function plt.annotate takes the text first, then the xy coordinate as a tuple.

2. Step 2: Check each option

   plt.annotate('Peak', xy=(2, 3)) uses plt.annotate('Peak', xy=(2, 3)) which is correct. Others misuse parameter names or functions.

3. Final Answer:

   plt.annotate('Peak', xy=(2, 3)) -> Option C

4. Quick Check:

   annotate(text, xy=point) = correct syntax [OK]

Hint: Use plt.annotate('text', xy=(x,y)) for annotations [OK]

Common Mistakes:

• Swapping text and xy parameters
• Using plt.text with wrong arguments
• Incorrect parameter names like label or text=

3. What will the following code display?

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], [4, 5, 6])
plt.annotate('Max', xy=(3, 6), xytext=(2, 5), arrowprops=dict(facecolor='black'))
plt.show()

medium

A. A line plot with an annotation 'Max' at point (3,6) with an arrow pointing from (2,5)

B. A line plot with text 'Max' at (2,5) but no arrow

C. A line plot with an annotation 'Max' at (2,5) without arrow

D. Syntax error due to incorrect arrowprops usage

Solution

1. Step 1: Understand plt.annotate parameters

   xy is the point to annotate (3,6), xytext is where the text appears (2,5), arrowprops adds an arrow from text to point.

2. Step 2: Analyze the code output

   The plot shows a line, with 'Max' text at (2,5) and an arrow pointing to (3,6).

3. Final Answer:

   A line plot with an annotation 'Max' at point (3,6) with an arrow pointing from (2,5) -> Option A

4. Quick Check:

   annotate with xy, xytext, arrowprops = arrowed label [OK]

Hint: xy=point, xytext=text location, arrowprops=arrow [OK]

Common Mistakes:

• Confusing xy and xytext roles
• Expecting arrow without arrowprops
• Thinking arrowprops causes syntax error

4. Identify the error in this code snippet:

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], [4, 5, 6])
plt.annotate('High', (3, 6), xytext=(2, 5), arrowprops='black')
plt.show()

medium

A. plt.plot arguments are incorrect

B. arrowprops should be a dictionary, not a string

C. The text 'High' should be passed as xytext

D. xytext parameter is missing

Solution

1. Step 1: Check arrowprops usage

   arrowprops must be a dictionary specifying arrow style, not a string.

2. Step 2: Verify other parameters

   xytext is present, text is first argument, plot args are correct.

3. Final Answer:

   arrowprops should be a dictionary, not a string -> Option B

4. Quick Check:

   arrowprops = dict(...) not string [OK]

Hint: arrowprops needs dict with arrow style, not string [OK]

Common Mistakes:

• Passing arrowprops as string
• Confusing text and xytext parameters
• Incorrect plot data format

5. You want to highlight the lowest point on a plot with an annotation that includes an arrow and custom text offset. Which code snippet correctly achieves this for data points x=[1,2,3], y=[5,3,6]?

hard

A. plt.annotate('Lowest', xy=(2, 3), xytext=(2, 3), arrowprops=dict(color='blue'))

B. plt.annotate('Lowest', xytext=(2, 3), xy=(1, 4), arrowprops='red')

C. plt.text('Lowest', xy=(2, 3), xytext=(1, 4), arrowprops=dict(facecolor='red'))

D. plt.annotate('Lowest', xy=(2, 3), xytext=(1, 4), arrowprops=dict(facecolor='red'))

Solution

1. Step 1: Identify the lowest point coordinates

   Lowest y is 3 at x=2, so xy=(2,3) is correct.

2. Step 2: Check annotation parameters

   xytext=(1,4) offsets text for clarity, arrowprops uses dict with facecolor for arrow color.

3. Step 3: Validate options

   plt.annotate('Lowest', xy=(2, 3), xytext=(1, 4), arrowprops=dict(facecolor='red')) matches correct syntax and logic. Others misuse parameters or functions.

4. Final Answer:

   plt.annotate('Lowest', xy=(2, 3), xytext=(1, 4), arrowprops=dict(facecolor='red')) -> Option D

5. Quick Check:

   Correct xy, xytext, arrowprops dict = plt.annotate('Lowest', xy=(2, 3), xytext=(1, 4), arrowprops=dict(facecolor='red')) [OK]

Hint: Use xy for point, xytext for label offset, arrowprops dict [OK]

Common Mistakes:

• Swapping xy and xytext
• Using plt.text instead of plt.annotate
• Passing arrowprops as string