Area chart with plt.fill_between in Matplotlib - Step-by-Step Execution

Concept Flow - Area chart with plt.fill_between

Prepare x and y data

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Call plt.fill_between(x, y1, y2)

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Matplotlib fills area between y1 and y2

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Display or save the plot

This flow shows how data arrays are prepared, then plt.fill_between fills the area between two y-values over x, and finally the plot is shown.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3, 4, 5]
y1 = [1, 3, 2, 5, 4]
plt.fill_between(x, y1, color='skyblue')
plt.show()

This code draws an area chart by filling the area under y1 over x with a sky blue color.

Execution Table

Step	Action	x values	y1 values	fill_between effect	Plot state
1	Prepare x and y1 data	[1, 2, 3, 4, 5]	[1, 3, 2, 5, 4]	No fill yet	Empty plot
2	Call plt.fill_between(x, y1)	[1, 2, 3, 4, 5]	[1, 3, 2, 5, 4]	Area under y1 filled with skyblue	Plot has filled area
3	Call plt.show()	[1, 2, 3, 4, 5]	[1, 3, 2, 5, 4]	Area displayed on screen	Plot visible to user

💡 Plot is displayed and script ends

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
x	undefined	[1, 2, 3, 4, 5]	[1, 2, 3, 4, 5]	[1, 2, 3, 4, 5]
y1	undefined	[1, 3, 2, 5, 4]	[1, 3, 2, 5, 4]	[1, 3, 2, 5, 4]
plot state	empty	empty	area filled	displayed

Key Moments - 2 Insights

Why does plt.fill_between fill the area under the curve and not above?

By default, plt.fill_between fills the area between y1 and y2=0 (the x-axis). Since y2 is not given, it assumes zero, so it fills under y1 as shown in step 2 of the execution_table.

What happens if x and y1 have different lengths?

Matplotlib will raise an error because x and y1 must be the same length to match points. This is why in step 1 both arrays have length 5.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what color is used to fill the area?

A green

B red

C skyblue

D no color

💡 Hint

Check the 'fill_between effect' column in step 2 of execution_table.

At which step does the plot become visible to the user?

A Step 1

B Step 3

C Step 2

D After script ends

💡 Hint

Look at the 'Plot state' column in execution_table.

If y1 was all zeros, what would plt.fill_between show?

A A filled area at zero height (flat line)

B No filled area

C An error

D A filled area above the x-axis

💡 Hint

Think about how fill_between fills area between y1 and zero baseline.

Concept Snapshot

plt.fill_between(x, y1, y2=0, color) fills the area between y1 and y2 over x.
If y2 is omitted, it defaults to zero (x-axis).
Use it to create area charts showing magnitude under curves.
Call plt.show() to display the plot.
x and y arrays must be same length.

Full Transcript

This visual execution shows how to create an area chart using matplotlib's plt.fill_between function. First, we prepare the x and y data arrays. Then, calling plt.fill_between fills the area between y1 and the x-axis with a chosen color. Finally, plt.show displays the plot on screen. The execution table traces these steps, showing variable values and plot state changes. Key moments clarify why the area fills under the curve and the importance of matching array lengths. The quiz tests understanding of color, display timing, and behavior with zero values.