Annotating specific points in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of annotation coordinates

What will be the coordinates of the annotation arrow tip in the plot after running this code?

Matplotlib

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

A (2, 4)

B (3, 6)

C (1, 2)

D (3, 5)

💡 Hint

The arrow points to the xy coordinate, not the text position.

The annotation arrow points to the xy parameter coordinates, which is (2, 4). The text is placed at xytext=(3, 5).

❓ data_output

intermediate

Number of annotations added

How many annotations will appear on the plot after running this code?

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3, 4]
y = [1, 4, 9, 16]
plt.plot(x, y)
for i in range(len(x)):
    if y[i] > 5:
        plt.annotate(f'High {y[i]}', xy=(x[i], y[i]), xytext=(x[i]+0.2, y[i]+1), arrowprops=dict(arrowstyle='->'))
plt.show()

A 1

B 3

C 2

D 4

💡 Hint

Count points where y is greater than 5.

Only y values 9 and 16 are greater than 5, so 2 annotations are added.

🔧 Debug

advanced

Identify the error in annotation code

What error will this code raise when run?

Matplotlib

import matplotlib.pyplot as plt
plt.scatter([1, 2], [3, 4])
plt.annotate('Test', xy=(1, 3), xytext=(2, 5), arrowprops='->')
plt.show()

A No error, plot shows annotation

B TypeError: arrowprops must be a dict

C ValueError: xytext must be a tuple

D SyntaxError: invalid syntax

💡 Hint

Check the type of arrowprops argument.

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

❓ visualization

advanced

Effect of annotation with offset points

Which option best describes the visual effect of this annotation code on the plot?

Matplotlib

import matplotlib.pyplot as plt
x = [0, 1, 2]
y = [0, 1, 4]
plt.plot(x, y, 'o-')
plt.annotate('Peak', xy=(2, 4), xytext=(2, 5), textcoords='offset points', arrowprops=dict(arrowstyle='-|>'))
plt.show()

A The text 'Peak' is placed 5 points above the point (2,4) with an arrow pointing to (2,4).

B The text 'Peak' is placed at (2,5) in data coordinates with no arrow.

C The text 'Peak' overlaps the point (2,4) with no offset or arrow.

D The text 'Peak' is placed 5 units above (2,4) in data coordinates with an arrow.

💡 Hint

textcoords='offset points' means offset in points, not data units.

Using textcoords='offset points' places text relative to the point in screen points, so text is 5 points above (2,4). Arrow points to (2,4).

🚀 Application

expert

Annotate maximum value in a dataset

Given a dataset, which code snippet correctly annotates the maximum y-value point with an arrow and label 'Max Value'?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np
x = np.linspace(0, 10, 100)
y = np.sin(x) + x/6
plt.plot(x, y)
# Insert annotation code here
plt.show()

A

max_val = max(y)
plt.annotate('Max Value', xy=(max_val, max_val), xytext=(max_val+1, max_val+1), arrowprops=dict(facecolor='red'))

B

max_idx = y.argmax()
plt.annotate('Max Value', xytext=(x[max_idx], y[max_idx]), xy=(x[max_idx]+1, y[max_idx]+1), arrowprops=dict(facecolor='red'))

C

max_idx = y.argmax()
plt.annotate('Max Value', xy=(x[max_idx], y[max_idx]), xytext=(x[max_idx]-1, y[max_idx]-1), arrowprops='->')

D

max_idx = y.argmax()
plt.annotate('Max Value', xy=(x[max_idx], y[max_idx]), xytext=(x[max_idx]+1, y[max_idx]+1), arrowprops=dict(facecolor='red'))

💡 Hint

Remember xy is the point annotated, xytext is text position, arrowprops must be dict.

Option D correctly finds max index, uses xy for point, xytext for label offset, and arrowprops as dict. Option D uses max value as coordinate incorrectly. Option D swaps xy and xytext. Option D uses string for arrowprops causing error.

Practice

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

easy

A. To increase the size of the plot

B. To change the color of the plot lines

C. To add labels or notes to specific points on the plot

D. To remove grid lines from the plot

Solution

1. Step 1: Understand what annotations do

   Annotations are used to add extra information like labels or notes to specific points on a plot.

2. Step 2: Compare options with the purpose

   Only To add labels or notes to specific points on the plot describes adding labels or notes to points, which matches the purpose of annotations.

3. Final Answer:

   To add labels or notes to specific points on the plot -> Option C

4. Quick Check:

   Annotations = labels/notes on points [OK]

Hint: Annotations label points to explain data clearly [OK]

Common Mistakes:

• Confusing annotations with changing plot colors
• Thinking annotations resize the plot
• Assuming annotations remove grid lines

2. Which of the following is the correct syntax to add an annotation at point (2, 5) with text 'Peak' in matplotlib?

easy

A. plt.annotate('Peak', xy=(2, 5))

B. plt.annotation('Peak', (2, 5))

C. plt.annotate('Peak', point=(2, 5))

D. plt.annotate(text='Peak', position=(2, 5))

Solution

1. Step 1: Recall the correct function and parameters

   The correct function is plt.annotate with parameters: text first, then xy for the point coordinates.

2. Step 2: Check each option's syntax

   plt.annotate('Peak', xy=(2, 5)) uses plt.annotate('Peak', xy=(2, 5)) which is correct. Others use wrong function names or parameter names.

3. Final Answer:

   plt.annotate('Peak', xy=(2, 5)) -> Option A

4. Quick Check:

   Correct syntax uses xy= for point [OK]

Hint: Use plt.annotate(text, xy=(x,y)) to label points [OK]

Common Mistakes:

• Using plt.annotation instead of plt.annotate
• Wrong parameter names like point or position
• Swapping text and coordinate order

3. What will the following code display?

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

medium

A. A plot with a point at (3,6) labeled 'High' with an arrow pointing from (2,5)

B. Syntax error due to wrong arrowprops usage

C. A plot with no labels or arrows

D. A plot with a point at (2,5) labeled 'High' with an arrow pointing from (3,6)

Solution

1. Step 1: Understand plt.annotate parameters

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

2. Step 2: Interpret the plot output

   The label 'High' appears at (3,6) with an arrow pointing to (2,5) on the plot line.

3. Final Answer:

   A plot with a point at (2,5) labeled 'High' with an arrow pointing from (3,6) -> Option D

4. Quick Check:

   xy = point, xytext = label position [OK]

Hint: xy=point, xytext=label position with arrow [OK]

Common Mistakes:

• Confusing xy and xytext roles
• Thinking arrow points from point to text
• Assuming no arrow appears without extra code

4. Identify the error in this code snippet that tries to annotate point (4, 7):

plt.plot([1,2,3,4], [3,5,2,7])
plt.annotate('Max', point=(4,7))
plt.show()

medium

A. plt.show() must come before plt.annotate

B. The parameter 'point' is invalid; should use 'xy' instead

C. plt.plot needs labels before annotation

D. The text 'Max' should be a number

Solution

1. Step 1: Check plt.annotate parameters

   The correct parameter for the point to annotate is 'xy', not 'point'.

2. Step 2: Identify the error cause

   Using 'point' causes a TypeError because plt.annotate does not recognize it.

3. Final Answer:

   The parameter 'point' is invalid; should use 'xy' instead -> Option B

4. Quick Check:

   Use xy= for annotation point [OK]

Hint: Use xy=, not point=, to specify annotation location [OK]

Common Mistakes:

• Using wrong parameter names like point or position
• Placing plt.show() before annotation
• Thinking annotation text must be numeric

5. You want to highlight the maximum value point on a plot with an annotation that places the label 10 units above the point and draws an arrow. Which code correctly achieves this for data x=[1,2,3], y=[4,9,7]?

hard

A. plt.plot(x, y)\nmax_idx = y.index(max(y))\nplt.annotate('Max', xy=(x[max_idx], y[max_idx]), xytext=(x[max_idx], y[max_idx]+10), arrowprops=dict(arrowstyle='->'))\nplt.show()

B. plt.plot(x, y)\nmax_idx = y.index(max(y))\nplt.annotate('Max', xytext=(x[max_idx], y[max_idx]), xy=(x[max_idx], y[max_idx]+10), arrowprops=dict(arrowstyle='->'))\nplt.show()

C. plt.plot(x, y)\nmax_val = max(y)\nplt.annotate('Max', xy=(max_val, x[max_idx]), xytext=(max_val, x[max_idx]+10), arrowprops=dict(arrowstyle='->'))\nplt.show()

D. plt.plot(x, y)\nmax_idx = y.index(max(y))\nplt.annotate('Max', xy=(x[max_idx], y[max_idx]), xytext=(x[max_idx], y[max_idx]-10), arrowprops=dict(arrowstyle='->'))\nplt.show()

Solution

1. Step 1: Find the max value index correctly

   max_idx = y.index(max(y)) finds the index of the highest y value.

2. Step 2: Set annotation point and label position

   xy is the max point (x[max_idx], y[max_idx]). xytext places label 10 units above: y[max_idx]+10.

3. Step 3: Use arrowprops to draw arrow

   arrowprops=dict(arrowstyle='->') correctly adds an arrow from label to point.

4. Final Answer:

   Code in option A correctly annotates max point with label 10 units above and arrow -> Option A

5. Quick Check:

   xy=point, xytext=above point, arrowprops=arrow [OK]

Hint: Use xytext=(x, y+offset) to place label above point [OK]

Common Mistakes:

• Swapping xy and xytext parameters
• Using wrong coordinates for annotation
• Placing label below instead of above
• Mixing x and y values in annotation