Area chart with plt.fill_between in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of plt.fill_between with simple data

What will be the output of this code snippet that uses plt.fill_between to create an area chart?

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3, 4]
y = [2, 3, 5, 7]
plt.fill_between(x, y, color='skyblue')
plt.show()

A An error because plt.fill_between requires three arguments

B A line plot connecting points (1,2), (2,3), (3,5), (4,7) without any shaded area

C A blue shaded area under the curve connecting points (1,2), (2,3), (3,5), (4,7)

D A scatter plot of points (1,2), (2,3), (3,5), (4,7) with blue dots

💡 Hint

Remember, plt.fill_between fills the area between the x-axis and the y-values.

The function plt.fill_between fills the area between the x-axis (default y=0) and the y-values given. Here, it creates a blue shaded area under the curve defined by points (x, y).

🧠 Conceptual

intermediate

Understanding the role of y2 in plt.fill_between

In plt.fill_between(x, y1, y2), what does the parameter y2 represent?

A The lower boundary of the filled area

B The upper boundary of the filled area

C The color of the filled area

D The x-axis values

💡 Hint

Think about the area being filled between two curves.

In plt.fill_between, y1 and y2 define the two boundaries between which the area is filled. y1 is the upper boundary by default, and y2 is the lower boundary. If y2 is omitted, it defaults to zero (the x-axis).

🔧 Debug

advanced

Identify the error in this plt.fill_between usage

What error will this code produce and why?

import matplotlib.pyplot as plt
x = [0, 1, 2, 3]
y1 = [1, 2, 3]
y2 = [0, 1, 1, 2]
plt.fill_between(x, y1, y2)
plt.show()

A IndexError because x is shorter than y2

B TypeError because y1 and y2 must be numpy arrays

C No error, the plot will display correctly

D ValueError due to mismatched lengths of x and y1

💡 Hint

Check if all input lists have the same length.

The lists x and y1 have different lengths (4 vs 3), which causes a ValueError because plt.fill_between requires all input arrays to have the same length.

❓ data_output

advanced

Resulting data from plt.fill_between with condition

Given this code, what is the number of points where the area is filled?

import numpy as np
import matplotlib.pyplot as plt
x = np.arange(5)
y1 = np.array([1, 3, 2, 5, 4])
y2 = np.array([0, 2, 1, 3, 2])
plt.fill_between(x, y1, y2, where=(y1 > y2))
plt.show()

A 0 points because y1 is never greater than y2

B 5 points where y1 is greater than y2

C 2 points where y1 is less than y2

D 3 points where y1 is greater than y2

💡 Hint

Compare y1 and y2 values element-wise.

All elements in y1 are greater than corresponding elements in y2, so the area is filled for all 5 points.

🚀 Application

expert

Visualizing overlapping areas with plt.fill_between

You want to visualize two overlapping datasets using plt.fill_between. Which code snippet correctly fills the area between two curves y1 and y2 with different colors for the overlapping and non-overlapping regions?

Matplotlib

import numpy as np
import matplotlib.pyplot as plt
x = np.linspace(0, 10, 100)
y1 = np.sin(x) + 1
y2 = 0.5 * np.cos(x) + 1

# Which option correctly colors overlapping and non-overlapping areas?

A

plt.fill_between(x, y1, y2, where=(y1 >= y2), color='blue', alpha=0.5)
plt.fill_between(x, y1, y2, where=(y1 < y2), color='red', alpha=0.5)
plt.show()

B

plt.fill_between(x, y1, y2, color='blue', alpha=0.5)
plt.fill_between(x, y2, y1, color='red', alpha=0.5)
plt.show()

C

plt.fill_between(x, y1, y2, where=(y1 > y2), color='red')
plt.fill_between(x, y1, y2, where=(y1 <= y2), color='blue')
plt.show()

D

plt.fill_between(x, y1, y2, color='green')
plt.fill_between(x, y2, y1, color='yellow')
plt.show()

💡 Hint

Use the 'where' parameter to separate regions based on condition.

Option A uses the 'where' parameter to fill the area between y1 and y2 with blue where y1 >= y2 and red where y1 < y2, correctly showing overlapping and non-overlapping areas with different colors.

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