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.