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

↓

Call plt.fill_between(x, y1, y2)

↓

Matplotlib fills area between y1 and y2

↓

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.

Practice

1. What does the plt.fill_between function do in matplotlib?

easy

A. It fills the area between two curves or a curve and a baseline.

B. It creates a scatter plot with filled markers.

C. It draws a bar chart with filled bars.

D. It adds grid lines to the plot.

Solution

1. Step 1: Understand the purpose of plt.fill_between

   The function plt.fill_between is designed to fill the area between two lines or between a line and a baseline, creating an area chart effect.

2. Step 2: Compare with other plot types

   Scatter plots, bar charts, and grid lines are created by different functions like plt.scatter, plt.bar, and plt.grid, not plt.fill_between.

3. Final Answer:

   It fills the area between two curves or a curve and a baseline. -> Option A

4. Quick Check:

   plt.fill_between fills area under curve [OK]

Hint: Remember: fill_between fills area under or between lines [OK]

Common Mistakes:

• Confusing fill_between with scatter or bar plot functions
• Thinking it adds grid lines
• Assuming it only fills under a single line without baseline

2. Which of the following is the correct syntax to fill the area between x and y values using matplotlib?

easy

A. plt.fill_between(y, x)

B. plt.fill_between(x, y, color='blue')

C. plt.fill(x, y)

D. plt.fill_between(x)

Solution

1. Step 1: Check the required parameters of plt.fill_between

   The function requires at least two arrays: x (horizontal axis) and y (vertical values). plt.fill_between(x, y, color='blue') correctly uses plt.fill_between(x, y, color='blue') which is valid syntax with an optional color argument.

2. Step 2: Identify incorrect options

   plt.fill(x, y) uses the incorrect function plt.fill instead of plt.fill_between. plt.fill_between(y, x) reverses the x and y parameters, which is incorrect. plt.fill_between(x) misses the y values, causing an error.

3. Final Answer:

   plt.fill_between(x, y, color='blue') -> Option B

4. Quick Check:

   Correct syntax includes x, y, and optional color [OK]

Hint: Syntax: plt.fill_between(x, y, color='color') [OK]

Common Mistakes:

• Using plt.fill instead of plt.fill_between
• Swapping x and y parameters
• Omitting y values

3. What will the following code produce?

import matplotlib.pyplot as plt
x = [1, 2, 3, 4]
y1 = [1, 3, 2, 4]
y2 = [0, 1, 1, 2]
plt.fill_between(x, y1, y2, color='green', alpha=0.5)
plt.show()

medium

A. A scatter plot of points from y1 and y2.

B. A green line plot of y1 with alpha transparency.

C. An error because plt.fill_between cannot take two y arrays.

D. A green shaded area between y1 and y2 curves with 50% transparency.

Solution

1. Step 1: Understand plt.fill_between with two y arrays

   When two y arrays are given, plt.fill_between fills the area between these two curves along the x-axis.

2. Step 2: Analyze color and alpha parameters

   The color is set to green and alpha=0.5 means 50% transparency, so the filled area will appear as a translucent green band between y1 and y2.

3. Final Answer:

   A green shaded area between y1 and y2 curves with 50% transparency. -> Option D

4. Quick Check:

   Two y arrays fill area between curves [OK]

Hint: Two y arrays fill area between curves with color and alpha [OK]

Common Mistakes:

• Thinking it plots lines instead of filled area
• Assuming alpha controls line thickness
• Believing plt.fill_between cannot take two y arrays

4. Identify the error in this code snippet:

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [2, 3]
plt.fill_between(x, y)
plt.show()

medium

A. plt.fill_between cannot be used without plt.plot first.

B. plt.fill_between requires a color argument.

C. x and y arrays have different lengths causing a ValueError.

D. No error; the code runs fine.

Solution

1. Step 1: Check array lengths for plt.fill_between

   The x array has 3 elements, but y has only 2. This mismatch causes a ValueError because the function expects arrays of equal length.

2. Step 2: Verify other requirements

   Color argument is optional, and plt.plot is not required before fill_between. So options B and C are incorrect.

3. Final Answer:

   x and y arrays have different lengths causing a ValueError. -> Option C

4. Quick Check:

   Array length mismatch causes error [OK]

Hint: Ensure x and y arrays have same length [OK]

Common Mistakes:

• Ignoring array length mismatch
• Thinking color is mandatory
• Assuming plt.plot must be called first

5. You want to create an area chart showing the difference between two datasets y1 and y2 over x, but only fill where y1 is greater than y2. Which code snippet correctly achieves this?

hard

A. plt.fill_between(x, y1, y2, where=(y1 > y2), color='blue', alpha=0.3)

B. plt.fill_between(x, y1, y2, color='blue', alpha=0.3)

C. plt.fill_between(x, y1, y2, where=(y2 > y1), color='blue', alpha=0.3)

D. plt.fill_between(x, y1, y2, where=(y1 == y2), color='blue', alpha=0.3)

Solution

1. Step 1: Use the 'where' parameter to conditionally fill

   The where argument in plt.fill_between allows filling only where a condition is true. To fill where y1 is greater than y2, use where=(y1 > y2).

2. Step 2: Check other options for correctness

   plt.fill_between(x, y1, y2, color='blue', alpha=0.3) fills everywhere between y1 and y2. plt.fill_between(x, y1, y2, where=(y2 > y1), color='blue', alpha=0.3) fills where y2 is greater, which is opposite. plt.fill_between(x, y1, y2, where=(y1 == y2), color='blue', alpha=0.3) fills only where y1 equals y2, which is usually no area.

3. Final Answer:

   plt.fill_between(x, y1, y2, where=(y1 > y2), color='blue', alpha=0.3) -> Option A

4. Quick Check:

   Use where=(y1 > y2) to fill selectively [OK]

Hint: Use where=(condition) to fill selectively [OK]

Common Mistakes:

• Omitting the where parameter for conditional fill
• Reversing the condition in where
• Using equality instead of greater than