Horizontal bar chart with plt.barh in Matplotlib - Step-by-Step Execution

Concept Flow - Horizontal bar chart with plt.barh

Prepare data: categories, values

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Call plt.barh(categories, values)

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Matplotlib draws horizontal bars

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Show or save the plot

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END

We prepare categories and their values, then call plt.barh to draw horizontal bars, and finally display the chart.

Execution Sample

Matplotlib

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

This code draws a horizontal bar chart with three categories and their values.

Execution Table

Step	Action	Input	Output/Effect
1	Prepare categories list	['A', 'B', 'C']	List ready for plotting
2	Prepare values list	[5, 3, 7]	Values ready for plotting
3	Call plt.barh(categories, values)	categories=['A','B','C'], values=[5,3,7]	Horizontal bars drawn: A=5, B=3, C=7
4	Call plt.show()	None	Plot window opens showing horizontal bar chart
5	User closes plot window	None	Execution ends

💡 Plot displayed and program ends after user closes the window

Variable Tracker

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

Key Moments - 3 Insights

Why do we pass categories first and values second in plt.barh?

plt.barh expects the first argument as the y-axis labels (categories) and the second as the bar lengths (values). This matches the horizontal layout where categories go vertically.

What happens if categories and values lists have different lengths?

Matplotlib will raise an error because each category needs a corresponding value. See execution_table step 3 where inputs must match in length.

Why do we call plt.show() after plt.barh?

plt.barh prepares the plot but does not display it. plt.show() opens the window to display the chart, as shown in execution_table step 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'values' after Step 2?

A ['A', 'B', 'C']

B [5, 3, 7]

C None

D [3, 5, 7]

💡 Hint

Check variable_tracker row for 'values' after Step 2

At which step does the horizontal bar chart actually get drawn?

A Step 3

B Step 1

C Step 4

D Step 5

💡 Hint

See execution_table action column for when plt.barh is called

If we swapped categories and values in plt.barh, what would happen?

A Matplotlib would raise an error immediately

B Plot would show correctly with no change

C Bars would be drawn with categories as lengths and values as labels

D Bars would be vertical instead of horizontal

💡 Hint

Recall plt.barh expects categories first, values second (see key_moments)

Concept Snapshot

plt.barh(categories, values) draws horizontal bars.
First argument: categories (y-axis labels).
Second argument: values (bar lengths).
Call plt.show() to display the plot.
Categories and values must be same length.
Useful for comparing quantities across categories horizontally.

Full Transcript

We start by preparing two lists: categories and values. Categories are labels like 'A', 'B', 'C'. Values are numbers like 5, 3, 7. Then we call plt.barh(categories, values) which draws horizontal bars for each category with length equal to its value. Finally, plt.show() displays the chart window. The variables categories and values hold the data throughout. It's important that categories come first and values second in plt.barh. If their lengths differ, matplotlib will raise an error. The plot only appears after plt.show() is called. This method is great for visualizing data horizontally, making category comparison easy.