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.

Practice

1. What does the marker parameter do in a matplotlib plot?

easy

A. It specifies the shape of the points shown on the plot.

B. It changes the color of the plot line.

C. It sets the thickness of the plot line.

D. It adds grid lines to the plot.

Solution

1. Step 1: Understand the role of the marker parameter

   The marker parameter controls how individual data points appear on the plot, such as circles, squares, or triangles.

2. Step 2: Differentiate marker from other parameters

   Other parameters like color or linewidth control line appearance, not point shapes.

3. Final Answer:

   It specifies the shape of the points shown on the plot. -> Option A

4. Quick Check:

   marker = point shape [OK]

Hint: Marker shapes show points clearly on plots [OK]

Common Mistakes:

• Confusing marker with color or line style
• Thinking marker adds grid lines
• Assuming marker changes line thickness

2. Which of the following is the correct way to plot data points with circle markers using matplotlib?

easy

A. plt.plot(x, y, marker='o')

B. plt.plot(x, y, marker='x')

C. plt.plot(x, y, marker='s')

D. plt.plot(x, y, marker='^')

Solution

1. Step 1: Recall marker symbols in matplotlib

   In matplotlib, 'o' is the symbol for circle markers.

2. Step 2: Match marker symbol to circle

   Other options: 'x' is cross, 's' is square, '^' is triangle up.

3. Final Answer:

   plt.plot(x, y, marker='o') -> Option A

4. Quick Check:

   Circle marker = 'o' [OK]

Hint: Circle marker is 'o' in matplotlib [OK]

Common Mistakes:

• Using 'x' or 's' for circle markers
• Forgetting to put marker in quotes
• Using marker parameter incorrectly

3. What will the following code display?

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

medium

A. A line plot with cross markers at each data point.

B. A scatter plot with circle markers.

C. A bar chart with triangle markers.

D. A line plot with triangle-up markers at each data point.

Solution

1. Step 1: Understand the code plot type

   The code uses plt.plot with x and y lists, so it creates a line plot.

2. Step 2: Interpret the marker parameter

   Marker '^' means triangle-up shape at each data point on the line.

3. Final Answer:

   A line plot with triangle-up markers at each data point. -> Option D

4. Quick Check:

   marker='^' means triangle-up markers [OK]

Hint: Marker '^' means triangle-up on line plot [OK]

Common Mistakes:

• Confusing plt.plot with scatter plot
• Misreading marker '^' as cross or circle
• Thinking it creates a bar chart

4. Identify the error in this code snippet:

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [3, 2, 1]
plt.plot(x, y, marker=o)
plt.show()

medium

A. The x and y lists have different lengths.

B. The marker value should be a string, so marker='o'.

C. plt.plot cannot use markers.

D. plt.show() is missing parentheses.

Solution

1. Step 1: Check marker parameter syntax

   The marker value must be a string, so it should be written as marker='o', not marker=o.

2. Step 2: Verify other parts of the code

   x and y lists have same length; plt.plot supports markers; plt.show() is correctly called.

3. Final Answer:

   The marker value should be a string, so marker='o'. -> Option B

4. Quick Check:

   Marker must be a string in quotes [OK]

Hint: Put marker symbols inside quotes [OK]

Common Mistakes:

• Forgetting quotes around marker symbol
• Assuming plt.plot can't use markers
• Misreading plt.show syntax

5. You want to plot two lines on the same graph with different markers: circles for the first line and squares for the second. Which code correctly does this?

hard

A. plt.plot(x1, y1, marker='x')
plt.plot(x2, y2, marker='^')
plt.show()

B. plt.plot(x1, y1, marker='s')
plt.plot(x2, y2, marker='o')
plt.show()

C. plt.plot(x1, y1, marker='o')
plt.plot(x2, y2, marker='s')
plt.show()

D. plt.plot(x1, y1, marker='o', marker='s')
plt.plot(x2, y2)
plt.show()

Solution

1. Step 1: Assign markers to each line separately

   Each call to plt.plot can have its own marker parameter to set point shapes.

2. Step 2: Match markers to lines as required

   First line uses 'o' for circles, second line uses 's' for squares.

3. Step 3: Check incorrect options

   plt.plot(x1, y1, marker='s') plt.plot(x2, y2, marker='o') plt.show() swaps markers; C uses wrong markers; D tries two markers in one call, which is invalid.

4. Final Answer:

   plt.plot(x1, y1, marker='o') plt.plot(x2, y2, marker='s') plt.show() -> Option C

5. Quick Check:

   Separate plot calls, each with correct marker [OK]

Hint: Use separate plot calls with different markers [OK]

Common Mistakes:

• Trying to set two markers in one plot call
• Swapping marker symbols by mistake
• Not calling plt.show() after plotting