Vertical bar chart with plt.bar in Matplotlib - Step-by-Step Execution

Concept Flow - Vertical bar chart with plt.bar

Prepare data: categories, values

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Call plt.bar(categories, values)

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Bars drawn vertically

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Add labels, title

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Show plot with plt.show()

This flow shows how data is prepared, passed to plt.bar to draw vertical bars, then labels and title are added before displaying the chart.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
categories = ['A', 'B', 'C']
values = [5, 7, 3]
plt.bar(categories, values)
plt.show()

This code draws a vertical bar chart with 3 bars labeled A, B, and C with heights 5, 7, and 3.

Execution Table

Step	Action	Input	Result
1	Prepare categories	['A', 'B', 'C']	Categories list ready
2	Prepare values	[5, 7, 3]	Values list ready
3	Call plt.bar	categories=['A','B','C'], values=[5,7,3]	Bars created at positions 0,1,2 with heights 5,7,3
4	Draw bars	Bars data	Vertical bars drawn on plot
5	Call plt.show	None	Plot window opens showing the bar chart

💡 Plot displayed and program waits for user to close window

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
categories	None	['A', 'B', 'C']	['A', 'B', 'C']	['A', 'B', 'C']	['A', 'B', 'C']
values	None	None	[5, 7, 3]	[5, 7, 3]	[5, 7, 3]

Key Moments - 2 Insights

Why do the bars appear at positions 0, 1, and 2 instead of the category names?

plt.bar uses the categories as labels on the x-axis but places bars at numeric positions 0, 1, 2 internally. The labels show under each bar as seen in step 3 of the execution_table.

What happens if the lengths of categories and values lists do not match?

Matplotlib will raise an error because each category needs a corresponding value. This is why in step 2 and 3 both lists have the same length.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what are the heights of the bars created?

A A, B, C

B 5, 7, 3

C 0, 1, 2

D None

💡 Hint

Check the 'Result' column in step 3 of execution_table for bar heights.

At which step does the plot window open to show the bar chart?

A Step 5

B Step 2

C Step 4

D Step 3

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table for when plt.show() is called.

If you change values to [2, 4, 6], how will the bars change in the execution_table?

A Categories will change

B Bars heights remain 5, 7, 3

C Bars heights will be 2, 4, 6 at step 3

D Plot will not show

💡 Hint

Refer to the 'Input' and 'Result' columns in step 3 of execution_table.

Concept Snapshot

plt.bar(categories, values) draws vertical bars.
Categories label x-axis positions 0,1,2...
Values set bar heights.
Call plt.show() to display.
Lengths of categories and values must match.

Full Transcript

This visual execution shows how to create a vertical bar chart using matplotlib's plt.bar function. First, we prepare two lists: categories for the x-axis labels and values for the bar heights. Then plt.bar is called with these lists, which draws vertical bars at positions 0, 1, 2 with heights from values. Finally, plt.show() opens a window displaying the chart. The execution table traces each step, showing how variables categories and values are set and used. Key moments clarify why bars are positioned numerically and the importance of matching list lengths. The quiz tests understanding of bar heights, display timing, and effect of changing values.