Grouped bar charts in Matplotlib - Step-by-Step Execution

Concept Flow - Grouped bar charts

Prepare data for groups and categories

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Calculate bar positions for each group

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Plot bars side-by-side for each category

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Add labels, title, legend

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Display the grouped bar chart

Grouped bar charts show multiple bars side-by-side for each group, allowing easy comparison across categories.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt

labels = ['A', 'B', 'C']
values1 = [5, 7, 3]
values2 = [6, 8, 4]

x = range(len(labels))

plt.bar([p - 0.2 for p in x], values1, width=0.4, label='Set 1')
plt.bar([p + 0.2 for p in x], values2, width=0.4, label='Set 2')

plt.xticks(x, labels)
plt.legend()
plt.show()

This code plots two sets of bars side-by-side for each label, creating a grouped bar chart.

Execution Table

Step	Action	Variable/Value	Effect/Output
1	Define labels	labels = ['A', 'B', 'C']	Labels for x-axis groups set
2	Define values1	values1 = [5, 7, 3]	First data set values stored
3	Define values2	values2 = [6, 8, 4]	Second data set values stored
4	Calculate x positions	x = range(3)	x = [0,1,2], positions for groups
5	Calculate positions for bars set 1	[p - 0.2 for p in x]	Positions: [-0.2, 0.8, 1.8] for first bars
6	Plot bars set 1	plt.bar([p - 0.2 for p in x], values1, width=0.4, label='Set 1')	Bars for Set 1 drawn at shifted left positions
7	Calculate positions for bars set 2	[p + 0.2 for p in x]	Positions: [0.2, 1.2, 2.2] for second bars
8	Plot bars set 2	plt.bar([p + 0.2 for p in x], values2, width=0.4, label='Set 2')	Bars for Set 2 drawn at shifted right positions
9	Set x-axis ticks	plt.xticks(x, labels)	Labels 'A','B','C' shown under groups
10	Add legend	plt.legend()	Legend shows 'Set 1' and 'Set 2' labels
11	Show plot	plt.show()	Grouped bar chart displayed
12	End	-	Execution complete

💡 All bars plotted and chart displayed, execution ends.

Variable Tracker

Variable	Start	After Step 4	After Step 5	After Step 7	Final
labels	undefined	['A', 'B', 'C']	['A', 'B', 'C']	['A', 'B', 'C']	['A', 'B', 'C']
values1	undefined	[5, 7, 3]	[5, 7, 3]	[5, 7, 3]	[5, 7, 3]
values2	undefined	[6, 8, 4]	[6, 8, 4]	[6, 8, 4]	[6, 8, 4]
x	undefined	[0, 1, 2]	[0, 1, 2]	[0, 1, 2]	[0, 1, 2]
positions set 1	undefined	undefined	[-0.2, 0.8, 1.8]	[-0.2, 0.8, 1.8]	[-0.2, 0.8, 1.8]
positions set 2	undefined	undefined	undefined	[0.2, 1.2, 2.2]	[0.2, 1.2, 2.2]

Key Moments - 3 Insights

Why do we subtract and add 0.2 to the x positions for the two sets of bars?

Subtracting 0.2 shifts the first set of bars slightly left, and adding 0.2 shifts the second set right, so bars don't overlap and appear side-by-side as shown in steps 5 and 7.

What does plt.xticks(x, labels) do in the grouped bar chart?

It sets the x-axis ticks at positions x = [0,1,2] and labels them with 'A', 'B', 'C' so viewers know which group each pair of bars belongs to, as in step 9.

Why do we use the same width (0.4) for both sets of bars?

Using the same width ensures bars are uniform in size and fit neatly side-by-side without overlapping, as seen in steps 6 and 8.

Visual Quiz - 3 Questions

Look at the execution table at step 5. What are the positions for the first set of bars?

A [-0.2, 0.8, 1.8]

B [0, 1, 2]

C [0.2, 1.2, 2.2]

D [0.4, 1.4, 2.4]

💡 Hint

Check the 'Variable/Value' column at step 5 in the execution_table.

At which step are the x-axis labels 'A', 'B', 'C' set under the bars?

A Step 6

B Step 7

C Step 9

D Step 10

💡 Hint

Look for plt.xticks call in the execution_table.

If we increase the bar width to 0.8 for both sets, what will likely happen?

A Bars will disappear

B Bars will overlap because total width exceeds group spacing

C Bars will be thinner and spaced out

D Bars will be colored differently

💡 Hint

Consider the width and position shifts in steps 5, 6, 7, and 8.

Concept Snapshot

Grouped bar charts show multiple bars side-by-side for each group.
Use position shifts (e.g., -0.2, +0.2) to separate bars.
Set bar width to fit bars neatly without overlap.
Use plt.xticks to label groups on x-axis.
Add legend to identify each data set.
Call plt.show() to display the chart.

Full Transcript

This visual execution traces how to create grouped bar charts using matplotlib. We start by defining labels and two sets of values. We calculate x positions for groups and shift them left and right to place bars side-by-side. Bars are plotted with the same width to avoid overlap. The x-axis ticks are set to show group labels, and a legend is added to distinguish data sets. Finally, the chart is displayed. Key points include shifting bar positions to group them and setting x-axis labels for clarity.