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

Concept Flow - Stacked bar charts

Prepare data for categories

↓

Plot first bar segment

↓

Plot next bar segment on top

↓

Repeat for all segments

↓

Display stacked bar chart

Stacked bar charts build bars by stacking segments for each category one on top of another, showing parts of a whole.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
categories = ['A', 'B', 'C']
values1 = [3, 5, 1]
values2 = [2, 3, 4]
plt.bar(categories, values1)
plt.bar(categories, values2, bottom=values1)
plt.show()

This code draws a stacked bar chart with two segments per category.

Execution Table

Step	Action	Data Used	Bottom Parameter	Bar Height Plotted	Visual Effect
1	Plot first bar segment	[3, 5, 1]	None	[3, 5, 1]	Bars of height 3,5,1 appear for A,B,C
2	Plot second bar segment	[2, 3, 4]	[3, 5, 1]	[2, 3, 4]	Bars stacked on top, total heights 5,8,5
3	Display chart	N/A	N/A	N/A	Stacked bar chart shown with two layers per category

💡 All segments plotted and chart displayed

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
categories	['A', 'B', 'C']	['A', 'B', 'C']	['A', 'B', 'C']	['A', 'B', 'C']
values1	[3, 5, 1]	[3, 5, 1]	[3, 5, 1]	[3, 5, 1]
values2	[2, 3, 4]	[2, 3, 4]	[2, 3, 4]	[2, 3, 4]
bottom	None	None	[3, 5, 1]	[3, 5, 1]

Key Moments - 3 Insights

Why do we use the 'bottom' parameter when plotting the second bar segment?

The 'bottom' parameter tells matplotlib where to start the next bar segment vertically, so it stacks on top of the first segment (see execution_table step 2). Without it, bars would overlap from zero.

What happens if the 'bottom' parameter is not the same length as the data?

Matplotlib will raise an error because it needs matching lengths to stack bars correctly. Each bar segment must align with the previous segment's height (refer to variable_tracker for lengths).

Can we stack more than two segments?

Yes! You keep adding bars with 'bottom' equal to the sum of all previous segments for each category, stacking them higher (concept_flow shows repeated stacking).

Visual Quiz - 3 Questions

Look at the execution_table, what is the 'bottom' parameter value when plotting the second bar segment?

A None

B [2, 3, 4]

C [3, 5, 1]

D [5, 8, 5]

💡 Hint

Check the 'Bottom Parameter' column in execution_table row 2

At which step do the bars first appear stacked on top of each other?

A Step 1

B Step 2

C Step 3

D Never stacked

💡 Hint

Look at the 'Visual Effect' column in execution_table rows 1 and 2

If we add a third segment with values [1, 2, 3], what should the 'bottom' parameter be for correct stacking?

A [5, 8, 5]

B [2, 3, 4]

C [3, 5, 1]

D [1, 2, 3]

💡 Hint

Sum the first two segments from variable_tracker values1 and values2

Concept Snapshot

Stacked bar charts show parts of categories stacked vertically.
Use plt.bar() for each segment.
Set 'bottom' to previous segments' heights.
Repeat for all segments.
Result: bars stacked to show totals and parts.

Full Transcript

Stacked bar charts build bars by stacking segments for each category one on top of another. First, we plot the base segment with plt.bar(). Then, for each next segment, we plot bars with the 'bottom' parameter set to the sum of all previous segments for that category. This stacks the bars visually. The example code plots two segments for categories A, B, and C. The execution table shows step-by-step how the bars are drawn and stacked. The variable tracker shows how data and the 'bottom' parameter change. Key moments clarify why 'bottom' is needed and how stacking works. The quiz tests understanding of stacking steps and parameters.