Marker size variation in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of varying marker sizes in scatter plot

What will be the output of the following code snippet that uses matplotlib to plot points with different marker sizes?

Matplotlib

import matplotlib.pyplot as plt
sizes = [20, 50, 80, 200]
x = [1, 2, 3, 4]
y = [10, 20, 25, 30]
plt.scatter(x, y, s=sizes)
plt.show()

A A scatter plot with 4 points, each point having marker sizes 20, 50, 80, and 200 respectively.

B A scatter plot with 4 points, but all markers are invisible because size is zero.

C A scatter plot with 4 points, all having the same marker size equal to the sum of sizes list.

D A scatter plot with 4 points, but marker sizes are ignored and default size is used.

💡 Hint

The 's' parameter in plt.scatter controls the marker size for each point individually if given a list.

The 's' argument accepts a list of sizes, so each point gets its own size. The plot will show points with sizes 20, 50, 80, and 200 respectively.

❓ data_output

intermediate

Number of points with marker size greater than 50

Given the following code, how many points have marker sizes greater than 50?

Matplotlib

import matplotlib.pyplot as plt
sizes = [10, 60, 45, 90, 30]
x = [1, 2, 3, 4, 5]
y = [5, 15, 10, 20, 25]
plt.scatter(x, y, s=sizes)
plt.show()

A 3

B 2

C 1

D 4

💡 Hint

Check the sizes list and count how many values are greater than 50.

Only the sizes 60 and 90 are greater than 50, so 2 points have marker sizes greater than 50.

🔧 Debug

advanced

Identify the error in marker size assignment

What error will this code raise when trying to plot marker sizes?

Matplotlib

import matplotlib.pyplot as plt
sizes = '20, 40, 60'
x = [1, 2, 3]
y = [5, 10, 15]
plt.scatter(x, y, s=sizes)
plt.show()

A TypeError: float() argument must be a string or a number, not 'tuple'

B No error, plot displays with marker sizes 20, 40, 60

C TypeError: 'str' object is not iterable

D ValueError: s must be a scalar or sequence of scalars

💡 Hint

Check the type of 'sizes' and what plt.scatter expects for 's'.

The 'sizes' variable is a string, not a list or array of numbers. plt.scatter expects a scalar or sequence of numbers for 's'. Passing a string causes a ValueError.

❓ visualization

advanced

Effect of marker size scaling on scatter plot

Which option correctly describes the visual difference when doubling the marker sizes in this scatter plot?

Matplotlib

import matplotlib.pyplot as plt
sizes = [30, 60, 90]
x = [1, 2, 3]
y = [10, 20, 30]
plt.scatter(x, y, s=[size * 2 for size in sizes])
plt.show()

A Markers appear the same size as original because scaling is ignored.

B Markers appear twice as large in diameter compared to original sizes.

C Markers appear twice as large in area compared to original sizes.

D Markers appear half the size of original because sizes are divided.

💡 Hint

Remember that 's' controls marker area, not diameter.

The 's' parameter controls the area of markers. Doubling 's' doubles the area, so markers appear larger but diameter increases by sqrt(2), not 2.

🚀 Application

expert

Create a scatter plot with marker sizes proportional to y-values

Which code snippet correctly creates a scatter plot where marker sizes are proportional to the y-values multiplied by 10?

A plt.scatter(x, y, s=[val * 10 for val in y])

B plt.scatter(x, y, s=y * 10)

C plt.scatter(x, y, s=[y * 10])

D plt.scatter(x, y, s=list(map(lambda x: x*10, x)))

💡 Hint

You need to multiply each y-value by 10 to get marker sizes.

Option A correctly multiplies each y-value by 10 using a list comprehension. Option A causes a TypeError because y is a list and cannot be multiplied directly. Option A creates a list with one element, not sizes for each point. Option A multiplies x-values, not y-values.

Practice

1. What does the s parameter control in the plt.scatter() function in matplotlib?

easy

A. The size of the markers in the scatter plot

B. The color of the markers

C. The shape of the markers

D. The transparency of the markers

Solution

1. Step 1: Understand the role of plt.scatter()

   The plt.scatter() function creates scatter plots where each point can have different visual properties.

2. Step 2: Identify the s parameter purpose

   The s parameter specifically sets the size of each marker in the scatter plot, either as a single value or a list of sizes.

3. Final Answer:

   The size of the markers in the scatter plot -> Option A

4. Quick Check:

   s controls marker size [OK]

Hint: Remember: 's' stands for size in scatter plots [OK]

Common Mistakes:

• Confusing 's' with color or shape parameters
• Thinking 's' controls transparency
• Assuming 's' changes marker style

2. Which of the following is the correct way to set marker sizes to 50 for all points in a scatter plot using matplotlib?

easy

A. plt.scatter(x, y, size=50)

B. plt.scatter(x, y, s=50)

C. plt.scatter(x, y, marker_size=50)

D. plt.scatter(x, y, sizes=50)

Solution

1. Step 1: Recall the parameter name for marker size

   The correct parameter to set marker size in plt.scatter() is s.

2. Step 2: Check the syntax for setting size

   Using s=50 sets all markers to size 50. Other options use incorrect parameter names.

3. Final Answer:

   plt.scatter(x, y, s=50) -> Option B

4. Quick Check:

   Use s for size [OK]

Hint: Use s to set size, not 'size' or 'sizes' [OK]

Common Mistakes:

• Using 'size' or 'sizes' instead of 's'
• Trying to set marker size with 'marker_size'
• Syntax errors from wrong parameter names

3. What will be the output marker sizes for the scatter plot from this code?

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [4, 5, 6]
sizes = [20, 50, 80]
plt.scatter(x, y, s=sizes)
plt.show()

medium

A. All markers will have size 20

B. Code will raise an error due to wrong size input

C. Markers will have default size because list is invalid

D. Markers will have sizes 20, 50, and 80 respectively

Solution

1. Step 1: Understand s with list input

   The s parameter accepts a list to assign different sizes to each marker.

2. Step 2: Match sizes to points

   Since sizes = [20, 50, 80], the first point gets size 20, second 50, third 80.

3. Final Answer:

   Markers will have sizes 20, 50, and 80 respectively -> Option D

4. Quick Check:

   List in s sets sizes per point [OK]

Hint: List in s sets sizes point-wise [OK]

Common Mistakes:

• Assuming all markers get the first size
• Thinking list input causes error
• Confusing s with color list

4. Identify the error in this code snippet that tries to vary marker sizes:

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [4, 5, 6]
sizes = [30, 60]
plt.scatter(x, y, s=sizes)
plt.show()

medium

A. The plt.scatter function is misspelled

B. The s parameter cannot accept lists

C. The sizes list length does not match x and y length

D. The variable sizes is not defined

Solution

1. Step 1: Check lengths of data and sizes

   The x and y lists have 3 points, but sizes list has only 2 values.

2. Step 2: Understand size list requirement

   The s list must match the number of points; otherwise, matplotlib raises a ValueError.

3. Final Answer:

   The sizes list length does not match x and y length -> Option C

4. Quick Check:

   Sizes list length must equal data points [OK]

Hint: Match sizes list length to data points [OK]

Common Mistakes:

• Ignoring size list length mismatch
• Thinking s can't be a list
• Assuming matplotlib auto-fills missing sizes

5. You want to create a scatter plot where marker sizes represent the population of cities. Given:

cities = ['A', 'B', 'C']
population = [1000, 5000, 3000]
x = [1, 2, 3]
y = [4, 5, 6]

Which code correctly scales the population to marker sizes and plots them?

hard

A. plt.scatter(x, y, s=[p*10 for p in population])

B. plt.scatter(x, y, s=[p/10 for p in population])

C. plt.scatter(x, y, s=population)

D. plt.scatter(x, y, s=[p**2 for p in population])

Solution

1. Step 1: Understand marker size scaling

   Population values are large; directly using them as sizes will create huge markers.

2. Step 2: Choose appropriate scaling

   Multiplying population by 10 scales sizes visibly but keeps them reasonable. Dividing by 10 makes markers too small; squaring makes them too large.

3. Final Answer:

   plt.scatter(x, y, s=[p*10 for p in population]) -> Option A

4. Quick Check:

   Scale sizes for visibility using multiplication [OK]

Hint: Scale data to reasonable marker sizes by multiplying [OK]

Common Mistakes:

• Using raw large values causing huge markers
• Dividing sizes too much making markers tiny
• Using squared sizes causing disproportionate markers