Horizontal grouped bars in Matplotlib

Introduction

Horizontal grouped bars help compare different groups side by side using horizontal bars. They make it easy to see differences across categories.

Comparing sales of different products across several months.

Showing survey results for multiple questions side by side.

Visualizing test scores of students in different subjects.

Comparing expenses across departments in a company.

Syntax

Matplotlib

import matplotlib.pyplot as plt

labels = ['Group1', 'Group2', 'Group3']
values1 = [10, 20, 30]
values2 = [15, 25, 35]

import numpy as np

bar_width = 0.35
indices = np.arange(len(labels))

plt.barh(indices, values1, bar_width, label='Set 1')
plt.barh(indices + bar_width, values2, bar_width, label='Set 2')

plt.yticks(indices + bar_width / 2, labels)
plt.legend()
plt.show()

Use barh() to create horizontal bars.

Shift the bars on the y-axis by bar width to group them side by side.

Examples

Two groups of horizontal bars with labels X, Y, Z on y-axis.

Matplotlib

plt.barh([0, 1, 2], [5, 10, 15], 0.4, label='A')
plt.barh([0.4, 1.4, 2.4], [7, 12, 17], 0.4, label='B')
plt.yticks([0.2, 1.2, 2.2], ['X', 'Y', 'Z'])
plt.legend()
plt.show()

Grouped bars comparing two years for fruit sales.

Matplotlib

labels = ['Apples', 'Bananas']
values1 = [30, 40]
values2 = [35, 45]
indices = np.arange(len(labels))
plt.barh(indices, values1, 0.3, label='2019')
plt.barh(indices + 0.3, values2, 0.3, label='2020')
plt.yticks(indices + 0.15, labels)
plt.legend()
plt.show()

Sample Program

This program shows student scores in three subjects for two years using horizontal grouped bars.

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

labels = ['Math', 'Science', 'English']
scores_2023 = [88, 92, 85]
scores_2024 = [90, 95, 87]

bar_width = 0.35
indices = np.arange(len(labels))

plt.barh(indices, scores_2023, bar_width, label='2023')
plt.barh(indices + bar_width, scores_2024, bar_width, label='2024')

plt.yticks(indices + bar_width / 2, labels)
plt.xlabel('Scores')
plt.title('Student Scores by Subject and Year')
plt.legend()
plt.tight_layout()
plt.show()

Important Notes

Make sure to shift the bars on the y-axis by the bar width to avoid overlap.

Use plt.yticks() to set the category labels in the middle of grouped bars.

Horizontal bars are useful when category names are long or many.

Summary

Horizontal grouped bars show side-by-side comparisons using horizontal bars.

Use barh() and shift bars on y-axis to group them.

Set y-axis ticks to label groups clearly.

Practice

1.

What is the main purpose of using horizontal grouped bars in a chart?

easy

A. To compare multiple categories side by side horizontally

B. To show data as a pie chart

C. To display data only vertically

D. To create a line graph

Solution

1. Step 1: Understand the chart type

   Horizontal grouped bars place bars side by side horizontally to compare groups.

2. Step 2: Identify the purpose

   This layout helps compare multiple categories clearly across groups.

3. Final Answer:

   To compare multiple categories side by side horizontally -> Option A

4. Quick Check:

   Horizontal grouped bars = compare categories horizontally [OK]

Hint: Grouped bars show side-by-side comparisons horizontally [OK]

Common Mistakes:

• Confusing horizontal bars with pie charts
• Thinking grouped bars are vertical only
• Mixing line graphs with bar charts

2.

Which of the following is the correct way to create horizontal grouped bars in matplotlib?

import matplotlib.pyplot as plt
import numpy as np

labels = ['A', 'B', 'C']
values1 = [5, 7, 3]
values2 = [6, 2, 4]

x = np.arange(len(labels))
width = 0.35

plt.barh(x, values1, height=width)
plt.barh(x + width, values2, height=width)
plt.show()

easy

A. Use plt.scatter for grouped bars

B. Use plt.bar with shifted positions for each group

C. Use plt.plot with x and y values

D. Use plt.barh with shifted positions for each group

Solution

1. Step 1: Check the function used

   The code uses plt.barh which creates horizontal bars.

2. Step 2: Verify grouping method

   Positions are shifted by adding width to x to group bars side by side horizontally.

3. Final Answer:

   Use plt.barh with shifted positions for each group -> Option D

4. Quick Check:

   plt.barh + shifted positions = grouped horizontal bars [OK]

Hint: Use plt.barh and shift positions to group bars horizontally [OK]

Common Mistakes:

• Using plt.bar instead of plt.barh for horizontal bars
• Not shifting positions causing overlapping bars
• Using plt.plot or plt.scatter which are not bar charts

3.

What will be the output of the following code?

import matplotlib.pyplot as plt
import numpy as np

labels = ['X', 'Y']
values1 = [3, 8]
values2 = [5, 2]

x = np.arange(len(labels))
width = 0.4

plt.barh(x, values1, height=width, color='blue')
plt.barh(x + width, values2, height=width, color='red')
plt.yticks(x + width / 2, labels)
plt.show()

medium

A. A line plot with two lines

B. Two vertical bar groups overlapping

C. Two sets of horizontal bars side by side for each label

D. A scatter plot with points at x positions

Solution

1. Step 1: Analyze barh usage

   plt.barh creates horizontal bars at positions x and x + width, shifted to group bars.

2. Step 2: Check y-axis labels

   yticks are set at middle positions between bars for labels 'X' and 'Y'.

3. Final Answer:

   Two sets of horizontal bars side by side for each label -> Option C

4. Quick Check:

   barh + shifted positions + ytick labels = grouped horizontal bars [OK]

Hint: Shift y positions and set yticks for grouped horizontal bars [OK]

Common Mistakes:

• Confusing horizontal bars with vertical bars
• Not setting yticks causing label misalignment
• Expecting line or scatter plot output

4.

Identify the error in this code for horizontal grouped bars:

import matplotlib.pyplot as plt
import numpy as np

labels = ['P', 'Q', 'R']
values1 = [4, 6, 5]
values2 = [7, 3, 8]

x = np.arange(len(labels))
width = 0.3

plt.barh(x, values1, height=width)
plt.barh(x, values2, height=width)
plt.yticks(x + width / 2, labels)
plt.show()

medium

A. Bars overlap because positions are not shifted for grouping

B. Height parameter is missing

C. yticks are set incorrectly

D. Values lists have different lengths

Solution

1. Step 1: Check bar positions

   Both plt.barh calls use the same x positions, so bars overlap instead of grouping.

2. Step 2: Verify grouping method

   Positions should be shifted by width to separate groups horizontally.

3. Final Answer:

   Bars overlap because positions are not shifted for grouping -> Option A

4. Quick Check:

   Same x positions without shift = overlapping bars [OK]

Hint: Shift bar positions to avoid overlap in grouped bars [OK]

Common Mistakes:

• Not shifting positions for second group
• Incorrect yticks placement
• Assuming height parameter fixes overlap

5.

You have sales data for 3 products over 2 years:

products = ['A', 'B', 'C']
year1 = [10, 15, 7]
year2 = [12, 9, 14]

How would you create a horizontal grouped bar chart to compare sales by product and year, ensuring product labels appear centered between grouped bars?

hard

A. Use plt.plot to draw lines for each product

B. Use plt.barh with positions shifted by width and set yticks at x + width/2

C. Use plt.barh with same positions for both years and set yticks at x

D. Use plt.bar with positions shifted by width and set xticks at x + width/2

Solution

1. Step 1: Use plt.barh with shifted positions

   Shift positions by width for each year to group bars horizontally side by side.

2. Step 2: Set yticks at middle of grouped bars

   Set yticks at x + width/2 so product labels appear centered between bars.

3. Final Answer:

   Use plt.barh with positions shifted by width and set yticks at x + width/2 -> Option B

4. Quick Check:

   Shift positions + yticks center = clear grouped horizontal bars [OK]

Hint: Shift bars and center yticks for clear grouped horizontal bars [OK]

Common Mistakes:

• Using vertical bars instead of horizontal
• Not shifting positions causing overlap
• Setting yticks incorrectly causing label misalignment