Text annotations in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of annotation with xytext and arrowprops

What will be the output of this code snippet that uses matplotlib to annotate a point with an arrow and custom text position?

Matplotlib

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.plot([1, 2, 3], [1, 4, 9])
ax.annotate('Square', xy=(2, 4), xytext=(3, 7), arrowprops=dict(facecolor='blue', shrink=0.05))
plt.show()

A A plot with points connected by a line and an annotation 'Square' pointing to (2,4) with an arrow starting at (3,7) in blue color.

B A plot with points connected by a line and an annotation 'Square' at (2,4) without any arrow.

C A plot with points connected by a line and an annotation 'Square' at (3,7) without any arrow.

D A plot with points connected by a line and an annotation 'Square' pointing to (3,7) with an arrow starting at (2,4) in red color.

💡 Hint

Look at the parameters xy and xytext in the annotate function and the color specified in arrowprops.

The annotate function places the text at xytext coordinates (3,7) and draws an arrow pointing to xy coordinates (2,4). The arrow color is set to blue by facecolor='blue'.

❓ data_output

intermediate

Number of annotations in a plot

Given the following code, how many annotations will be visible on the plot?

Matplotlib

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.plot([0, 1, 2], [0, 1, 4])
ax.annotate('Start', xy=(0, 0))
ax.annotate('Middle', xy=(1, 1), xytext=(1, 2))
ax.annotate('End', xy=(2, 4), xytext=(2, 5), arrowprops=dict(arrowstyle='->'))
plt.show()

A 2

B 3

C 1

D 0

💡 Hint

Each call to annotate adds one annotation regardless of arrow presence.

There are three calls to annotate, so three annotations appear: 'Start' at (0,0), 'Middle' at (1,2), and 'End' at (2,5) with an arrow pointing to (2,4).

🔧 Debug

advanced

Identify the error in annotation code

What error will this code raise when executed?

Matplotlib

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.plot([1, 2, 3], [1, 4, 9])
ax.annotate('Point', xy=(2, 4), xytext=(3, 7), arrowprops={'color': 'green'})
plt.show()

A No error, the plot shows annotation with a green arrow.

B SyntaxError due to incorrect dictionary syntax in arrowprops.

C KeyError because 'color' is not a valid key in arrowprops.

D TypeError because arrowprops must be a list, not a dict.

💡 Hint

Check the valid keys for arrowprops in matplotlib annotations.

The arrowprops dictionary does not accept 'color' as a key. The correct key to set arrow color is facecolor or edgecolor. Using 'color' causes a KeyError.

❓ visualization

advanced

Effect of different arrow styles in annotations

Which option shows the correct arrow style for the annotation with arrowstyle='fancy'?

Matplotlib

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.plot([0, 1], [0, 1])
ax.annotate('Fancy Arrow', xy=(1, 1), xytext=(0, 0), arrowprops=dict(arrowstyle='fancy'))
plt.show()

A An arrow with a double-headed style connecting (0,0) to (1,1).

B A simple straight arrow with a line and a triangular head connecting (0,0) to (1,1).

C No arrow is drawn, only the text at (0,0).

D An arrow with a filled triangular head and a curved tail connecting (0,0) to (1,1).

💡 Hint

The fancy arrow style has a filled head and a curved tail.

The fancy arrow style draws an arrow with a filled triangular head and a curved tail, making it visually distinct from a simple arrow.

🧠 Conceptual

expert

Understanding coordinate systems in annotations

In matplotlib annotations, what coordinate system does the xycoords='axes fraction' parameter refer to?

A Coordinates are given as a fraction of the axes width and height, ranging from 0 to 1.

B Coordinates are given in data units of the plotted data.

C Coordinates are given relative to the entire screen resolution.

D Coordinates are given in pixels relative to the figure window.

💡 Hint

Think about how fractions relate to the axes size.

The axes fraction coordinate system means coordinates are fractions of the axes dimensions, where (0,0) is bottom-left and (1,1) is top-right of the axes area.

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