Lollipop charts in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a basic lollipop chart code

What will be the output of this Python code that creates a lollipop chart using matplotlib?

Matplotlib

import matplotlib.pyplot as plt

categories = ['A', 'B', 'C', 'D']
values = [4, 7, 1, 8]

plt.stem(categories, values, basefmt=" ", use_line_collection=True)
plt.show()

A A vertical lollipop chart with stems from zero to values for categories A, B, C, D

B A horizontal bar chart with bars for categories A, B, C, D

C A scatter plot with points at (categories, values) but no stems

D A line plot connecting points for categories A, B, C, D

💡 Hint

Look at the plt.stem function and what it draws.

The plt.stem function creates vertical lines (stems) from a baseline to the data points, with markers at the data points, forming a lollipop chart.

❓ data_output

intermediate

Number of stems in a lollipop chart

Given this code, how many stems (lines) will the lollipop chart display?

Matplotlib

import matplotlib.pyplot as plt

labels = ['X', 'Y', 'Z']
values = [10, 5, 15]

plt.stem(labels, values, basefmt=" ", use_line_collection=True)
plt.show()

A 1

B 0

C 3

D 15

💡 Hint

Count how many data points are plotted.

Each data point corresponds to one stem line in the lollipop chart, so the number of stems equals the number of values.

🔧 Debug

advanced

Identify the error in lollipop chart code

What error will this code raise when run?

Matplotlib

import matplotlib.pyplot as plt

categories = ['A', 'B', 'C']
values = [5, 10]

plt.stem(categories, values, basefmt=" ", use_line_collection=True)
plt.show()

A No error, chart displays correctly

B TypeError: categories must be numeric

C SyntaxError: invalid syntax

D ValueError: x and y must have same length

💡 Hint

Check if the lengths of categories and values match.

The plt.stem function requires x and y to have the same length. Here categories has 3 items but values has 2, causing a ValueError.

❓ visualization

advanced

Effect of changing baseline in lollipop chart

What will be the visual effect of setting basefmt='r-' in plt.stem for this code?

Matplotlib

import matplotlib.pyplot as plt

labels = ['P', 'Q', 'R']
values = [3, 6, 9]

plt.stem(labels, values, basefmt='r-', use_line_collection=True)
plt.show()

A The baseline line will be red and solid under the stems

B The stems will be red instead of default color

C The markers at data points will be red circles

D The chart will have no baseline line visible

💡 Hint

basefmt controls the baseline line style and color.

The basefmt parameter sets the style of the baseline line. 'r-' means a red solid line under the stems.

🚀 Application

expert

Choosing correct code for horizontal lollipop chart

Which option produces a horizontal lollipop chart with categories on y-axis and values on x-axis?

A

plt.stem(labels, values, basefmt=' ', use_line_collection=True)
plt.gca().invert_yaxis()
plt.show()

B

plt.stem(values, labels, basefmt=' ', orientation='horizontal', use_line_collection=True)
plt.show()

C

plt.stem(values, labels, basefmt=' ', use_line_collection=True)
plt.show()

D

plt.stem(labels, values, basefmt=' ', orientation='horizontal', use_line_collection=True)
plt.show()

💡 Hint

Check the orientation parameter and axis order for horizontal stem plots.

To create a horizontal lollipop chart, plt.stem must have orientation='horizontal' and x, y data swapped: x=values, y=labels. Option B correctly sets orientation and axis order.

Practice

1. What is the main purpose of a lollipop chart in data visualization?

easy

A. To compare values using lines and dots for clear differences

B. To show the distribution of data using bars

C. To display data points on a map

D. To create 3D surface plots

Solution

1. Step 1: Understand lollipop chart basics

   A lollipop chart uses lines and dots to compare values clearly, making differences easy to see.

2. Step 2: Compare with other chart types

   Bar charts show distribution, maps show locations, and 3D plots show surfaces, which are different from lollipop charts.

3. Final Answer:

   To compare values using lines and dots for clear differences -> Option A

4. Quick Check:

   Lollipop chart = lines + dots comparison [OK]

Hint: Remember: lollipop = line + dot comparison [OK]

Common Mistakes:

• Confusing lollipop charts with bar charts
• Thinking lollipop charts show geographic data
• Assuming lollipop charts are 3D plots

2. Which matplotlib function is used to create a basic lollipop chart?

easy

A. plt.bar()

B. plt.scatter()

C. plt.plot()

D. plt.stem()

Solution

1. Step 1: Identify the function for lollipop charts

   Matplotlib's plt.stem() creates lollipop charts by drawing lines with markers at the ends.

2. Step 2: Differentiate from other plotting functions

   plt.bar() makes bar charts, plt.scatter() makes scatter plots, and plt.plot() makes line plots, not lollipop charts.

3. Final Answer:

   plt.stem() -> Option D

4. Quick Check:

   Use plt.stem() for lollipop charts [OK]

Hint: Use plt.stem() to combine lines and dots [OK]

Common Mistakes:

• Using plt.bar() instead of plt.stem()
• Confusing scatter plots with lollipop charts
• Trying plt.plot() which lacks markers by default

3. What will the following code output?

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [4, 2, 5]
plt.stem(x, y)
plt.show()

medium

A. A bar chart with bars at x=1,2,3 and heights 4,2,5

B. A lollipop chart with stems at x=1,2,3 and heights 4,2,5

C. A scatter plot with points at (1,4), (2,2), (3,5)

D. An error because plt.stem() needs more arguments

Solution

1. Step 1: Understand plt.stem() behavior

   The function plt.stem(x, y) draws vertical lines (stems) from zero to each y value at positions x, with dots at the top.

2. Step 2: Analyze the given data points

   Here, x = [1,2,3] and y = [4,2,5], so stems appear at x=1 with height 4, x=2 with height 2, and x=3 with height 5.

3. Final Answer:

   A lollipop chart with stems at x=1,2,3 and heights 4,2,5 -> Option B

4. Quick Check:

   plt.stem(x,y) = lollipop chart [OK]

Hint: plt.stem(x,y) draws stems at x with heights y [OK]

Common Mistakes:

• Thinking plt.stem() creates bar charts
• Confusing scatter plot with lollipop chart
• Assuming plt.stem() requires extra arguments

4. Identify the error in this code for creating a lollipop chart:

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [4, 5]
plt.stem(x, y)
plt.show()

medium

A. plt.stem() is not imported correctly

B. plt.show() is missing parentheses

C. x and y have different lengths causing a ValueError

D. No error, code runs fine

Solution

1. Step 1: Check lengths of x and y

   x has 3 elements, y has 2 elements. They must be the same length for plotting.

2. Step 2: Understand matplotlib error

   Matplotlib raises a ValueError when x and y lengths differ because it cannot pair points correctly.

3. Final Answer:

   x and y have different lengths causing a ValueError -> Option C

4. Quick Check:

   Equal length x,y needed [OK]

Hint: Check x and y lengths match before plotting [OK]

Common Mistakes:

• Ignoring length mismatch between x and y
• Thinking plt.show() needs no parentheses
• Assuming plt.stem() import causes error

5. Which code snippet correctly changes the marker color to red?

import matplotlib.pyplot as plt
x = [1, 2, 3, 4]
y = [5, 3, 9, 6]
markerline, stemlines, baseline = plt.stem(x, y)
# Change marker color to red
# Which line below is correct?

hard

A. markerline.set_color('red')

B. markerline.get_markerfacecolor()[2].set_color('red')

C. markerline.get_markerfacecolor()[2] = 'red'

D. markerline.get_markerfacecolor()[2].set_facecolor('red')

Solution

1. Step 1: Understand plt.stem() return values

   It returns markerline, stemlines, baseline. markerline is a Line2D object representing all markers.

2. Step 2: Change marker color correctly

   Use markerline.set_color('red') to change the marker colors to red.

3. Final Answer:

   markerline.set_color('red') -> Option A

4. Quick Check:

   Use set_color() on markerline to change marker colors [OK]

Hint: Use set_color() on markerline to change marker colors [OK]

Common Mistakes:

• Trying to assign color directly to a list element
• Using get_markerfacecolor() incorrectly
• Confusing set_color() with set_facecolor()