Horizontal grouped bars in Matplotlib - Step-by-Step Execution

Concept Flow - Horizontal grouped bars

Prepare data for groups and categories

↓

Calculate bar positions for each group

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Plot horizontal bars for each category side-by-side

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Add labels, legend, and display plot

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END

The flow shows preparing data, calculating positions for grouped bars, plotting horizontal bars side-by-side, then labeling and displaying the chart.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt

labels = ['G1', 'G2', 'G3']
cat1 = [5, 7, 3]
cat2 = [6, 2, 4]

bar_width = 0.35
indices = range(len(labels))

plt.barh([i for i in indices], cat1, height=bar_width, label='Cat1')
plt.barh([i + bar_width for i in indices], cat2, height=bar_width, label='Cat2')

plt.yticks([i + bar_width/2 for i in indices], labels)
plt.legend()
plt.show()

This code plots two categories of horizontal bars grouped by labels G1, G2, G3 side-by-side.

Execution Table

Step	Action	Variable/Value	Effect/Output
1	Define labels	labels = ['G1', 'G2', 'G3']	Groups to plot
2	Define category values	cat1 = [5,7,3], cat2 = [6,2,4]	Data for bars
3	Set bar width	bar_width = 0.35	Thickness of each bar
4	Calculate indices	indices = [0,1,2]	Positions for groups
5	Plot cat1 bars	plt.barh(indices, cat1, height=0.35)	Bars at positions 0,1,2 with cat1 values
6	Plot cat2 bars	plt.barh([i+0.35 for i in indices], cat2, height=0.35)	Bars shifted up by 0.35 for cat2
7	Set y-ticks	plt.yticks([i+0.175 for i in indices], labels)	Labels centered between grouped bars
8	Add legend	plt.legend()	Shows category labels
9	Show plot	plt.show()	Displays horizontal grouped bar chart
10	End	-	Plot complete

💡 All bars plotted and displayed, execution ends.

Variable Tracker

Variable	Start	After Step 2	After Step 4	After Step 6	Final
labels	undefined	['G1', 'G2', 'G3']	['G1', 'G2', 'G3']	['G1', 'G2', 'G3']	['G1', 'G2', 'G3']
cat1	undefined	[5,7,3]	[5,7,3]	[5,7,3]	[5,7,3]
cat2	undefined	[6,2,4]	[6,2,4]	[6,2,4]	[6,2,4]
bar_width	undefined	undefined	0.35	0.35	0.35
indices	undefined	undefined	[0,1,2]	[0,1,2]	[0,1,2]
cat2_positions	undefined	undefined	undefined	[0.35,1.35,2.35]	[0.35,1.35,2.35]

Key Moments - 3 Insights

Why do we add bar_width to indices for the second category bars?

Adding bar_width shifts the second category bars up so they don't overlap the first category bars, creating side-by-side grouping as shown in execution_table step 6.

How do y-ticks align with grouped bars?

Y-ticks are set at indices plus half the bar_width (step 7) to center labels between the two bars in each group.

What happens if bar_width is too large?

Bars may overlap or labels may misalign. The bar_width controls thickness and spacing, so it must be chosen carefully as in step 3.

Visual Quiz - 3 Questions

Look at the execution_table at step 6, what are the y positions for the second category bars?

A [0.35, 1.35, 2.35]

B [0.175, 1.175, 2.175]

C [0, 1, 2]

D [1, 2, 3]

💡 Hint

Check the 'Variable/Value' column at step 6 in execution_table for cat2 bar positions.

According to variable_tracker, what is the value of 'indices' after step 4?

A [1, 2, 3]

B [0, 1, 2]

C [0.35, 1.35, 2.35]

D undefined

💡 Hint

Look at the 'indices' row under 'After Step 4' in variable_tracker.

If bar_width was changed to 0.5, how would the y-ticks positions change?

A They would be at indices + 0.5

B They would stay at indices

C They would be at indices + 0.25

D They would be at indices + 1

💡 Hint

Y-ticks are set at indices plus half the bar_width as shown in execution_table step 7.

Concept Snapshot

Horizontal grouped bars:
- Use plt.barh() for horizontal bars
- Calculate positions for each group and category
- Shift bars by bar_width to group side-by-side
- Set y-ticks centered between bars
- Add legend and show plot
- Controls thickness and spacing with bar_width

Full Transcript

This visual execution traces how to create horizontal grouped bar charts using matplotlib. We start by defining group labels and two categories of data. We set a bar width to control thickness. We calculate indices for group positions. We plot the first category bars at these indices. Then we plot the second category bars shifted up by the bar width to avoid overlap. Y-ticks are set centered between the grouped bars for clear labeling. Finally, we add a legend and display the plot. Variables like labels, cat1, cat2, bar_width, and indices change step-by-step as shown. Key points include shifting bars to group them side-by-side and centering labels. The quiz tests understanding of bar positions and label alignment.