Markers on data points in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of marker style in matplotlib scatter plot

What is the output of this code snippet that plots points with different markers?

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [4, 5, 6]
plt.scatter(x, y, marker='^')
plt.show()

A A scatter plot with star markers at points (1,4), (2,5), (3,6)

B A scatter plot with circle markers at points (1,4), (2,5), (3,6)

C A scatter plot with square markers at points (1,4), (2,5), (3,6)

D A scatter plot with triangle-up markers at points (1,4), (2,5), (3,6)

💡 Hint

The marker parameter controls the shape of points in scatter plots.

The marker '^' in matplotlib means triangle-up markers. So the points are shown as triangles pointing upwards.

❓ data_output

intermediate

Count of unique marker types in a plot

Given this code, how many unique marker types are used in the plot?

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3, 4]
y = [1, 4, 9, 16]
markers = ['o', 's', '^', 'o']
for xi, yi, m in zip(x, y, markers):
    plt.scatter(xi, yi, marker=m)
plt.show()

A 3

B 4

C 2

D 1

💡 Hint

Look at the list of markers and count distinct symbols.

The markers list has 'o', 's', '^', and 'o'. The unique markers are 'o', 's', and '^', so 3 unique types.

🔧 Debug

advanced

Identify the error in marker usage

What error does this code raise when run?

Matplotlib

import matplotlib.pyplot as plt
plt.plot([1, 2, 3], [4, 5, 6], marker='z')
plt.show()

A SyntaxError: invalid syntax

B ValueError: Unrecognized marker style 'z'

C No error, plots with default marker

D TypeError: marker must be a string

💡 Hint

Check if 'z' is a valid marker in matplotlib.

The marker 'z' is not a valid marker style in matplotlib, so it raises a ValueError.

❓ visualization

advanced

Visual difference of marker sizes

Which option shows the plot with the largest marker size for the point at (2, 3)?

Matplotlib

import matplotlib.pyplot as plt
plt.scatter([1, 2, 3], [2, 3, 4], marker='o', s=50)
plt.scatter([2], [3], marker='o', s=200)
plt.show()

A The point at (2, 3) is the largest circle marker on the plot

B All points have the same marker size

C The point at (3, 4) is the largest marker

D The point at (1, 2) is the largest marker

💡 Hint

The 's' parameter controls marker size in scatter plots.

The point at (2, 3) is plotted twice, once with size 50 and once with size 200, so the larger marker is at (2, 3).

🚀 Application

expert

Choosing markers for categorical data visualization

You want to plot three categories of data points with distinct markers for clarity. Which set of markers is best to ensure clear visual distinction?

A ['o', 'o', 'o'] # all circles

B ['.', ',', '_'] # point, pixel, horizontal line

C ['o', 's', '^'] # circle, square, triangle-up

D ['x', 'x', 'x'] # all crosses

💡 Hint

Use distinct shapes to differentiate categories clearly.

Option C uses three different marker shapes, making categories visually distinct. Other options use identical or very similar markers.