Time series with fill_between in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this fill_between plot code?

Consider the following Python code using matplotlib to plot a time series with shaded area between two lines. What will be the color of the shaded area?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

days = np.arange(5)
line1 = np.array([1, 3, 2, 5, 4])
line2 = np.array([0, 2, 1, 3, 2])

plt.fill_between(days, line1, line2, color='green', alpha=0.3)
plt.show()

A A green shaded area with full opacity between the two lines

B A blue shaded area with 30% opacity between the two lines

C No shaded area because fill_between requires y1 > y2 always

D A green shaded area with 30% opacity between the two lines

💡 Hint

Check the color and alpha parameters in fill_between.

The fill_between function fills the area between line1 and line2 with the specified color and transparency. Here, color is green and alpha is 0.3, so the shaded area is green with 30% opacity.

❓ data_output

intermediate

How many points are shaded in this fill_between example?

Given the following code, how many points will be shaded between the two time series?

Matplotlib

import numpy as np
import matplotlib.pyplot as plt

days = np.arange(10)
line1 = np.sin(days / 2) + 2
line2 = np.cos(days / 2) + 1

plt.fill_between(days, line1, line2)
plt.show()

A 11 points are shaded, including start and end

B 10 points are shaded, one for each day

C 9 points are shaded, because fill_between shades between intervals

D No points are shaded because line1 is always below line2

💡 Hint

Remember that fill_between shades between intervals, not just points.

The fill_between function shades the area between points, so for 10 points, there are 9 intervals shaded.

🔧 Debug

advanced

What error does this fill_between code raise?

What error will this code produce?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

days = np.arange(5)
line1 = np.array([1, 2, 3])
line2 = np.array([0, 1, 2, 3, 4])

plt.fill_between(days, line1, line2)
plt.show()

A ValueError due to mismatched array lengths

B TypeError because days is not a list

C No error, plot shows with missing points

D IndexError due to out of range access

💡 Hint

Check if all arrays have the same length.

The fill_between function requires all input arrays to have the same length. Here, line1 has length 3 but days and line2 have length 5, causing a ValueError.

🚀 Application

advanced

Which option produces a plot with fill_between highlighting values above a threshold?

You want to highlight the area where a time series is above 0.5 using fill_between. Which code snippet does this correctly?

A plt.fill_between(x, y, 0.5, where=(y < 0.5), color='red', alpha=0.4)

B plt.fill_between(x, y, 0.5, where=(y > 0.5), color='red', alpha=0.4)

C plt.fill_between(x, y, 0.5, color='red', alpha=0.4)

D plt.fill_between(x, y, 0.5, where=(y == 0.5), color='red', alpha=0.4)

💡 Hint

Use the where parameter to specify the condition.

Option B correctly uses where=(y > 0.5) to fill only where the series is above 0.5. Other options fill incorrectly or fill everywhere.

🧠 Conceptual

expert

What is the effect of using fill_between with negative alpha values?

What happens if you set the alpha parameter in fill_between to a negative value like -0.5?

A The fill is fully transparent, so no shading is visible

B Matplotlib raises a ValueError because alpha must be between 0 and 1

C The fill is rendered with 50% opacity ignoring the negative sign

D The fill color is inverted due to negative alpha

💡 Hint

Matplotlib clamps alpha values outside [0, 1] to the valid range.

The alpha parameter controls transparency. Matplotlib clamps values outside [0, 1] to that range. A negative value like -0.5 is clamped to 0, making the fill fully transparent so no shading is visible.

Practice

1. What does the fill_between function do in a time series plot using matplotlib?

easy

A. Creates a bar chart from time series data

B. Plots scatter points on the time series

C. Colors the area between two lines to highlight differences or ranges

D. Removes grid lines from the plot

Solution

1. Step 1: Understand the purpose of fill_between

   The fill_between function is used to color the area between two lines or curves in a plot.

2. Step 2: Connect to time series visualization

   In time series plots, it highlights differences or ranges between two data series by filling the area between them.

3. Final Answer:

   Colors the area between two lines to highlight differences or ranges -> Option C

4. Quick Check:

   fill_between colors area between lines = A [OK]

Hint: Remember fill_between fills area between lines visually [OK]

Common Mistakes:

• Confusing fill_between with scatter plot
• Thinking it creates bar charts
• Assuming it removes grid lines

2. Which of the following is the correct syntax to fill the area between two time series arrays y1 and y2 over time points x using matplotlib?

easy

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

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

C. plt.fill_between(y1, y2, x, color='blue')

D. plt.fill_between(x, y1, y2, linestyle='--')

Solution

1. Step 1: Check function parameters order

   The correct order for fill_between is x (time points), then y1 and y2 (values to fill between).

2. Step 2: Validate optional arguments

   Color and transparency (alpha) are valid optional arguments; linestyle is not used for fill_between.

3. Final Answer:

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

4. Quick Check:

   fill_between(x, y1, y2) with color and alpha = C [OK]

Hint: Remember order: x, y1, y2 in fill_between [OK]

Common Mistakes:

• Swapping y1 and y2 with x
• Using linestyle instead of color or alpha
• Omitting one of the y arrays

3. What will be the output of this code snippet?

import matplotlib.pyplot as plt
import numpy as np
x = np.arange(5)
y1 = np.array([1, 3, 2, 5, 4])
y2 = np.array([2, 4, 3, 6, 5])
plt.plot(x, y1, label='y1')
plt.plot(x, y2, label='y2')
plt.fill_between(x, y1, y2, color='green', alpha=0.4)
plt.legend()
plt.show()

medium

A. An error because fill_between requires y1 > y2

B. A plot with two lines but no shaded area

C. A plot with only one line and a green shaded area below it

D. A plot with two lines and a green shaded area between them

Solution

1. Step 1: Understand plot and fill_between behavior

   The code plots two lines y1 and y2 over x, then fills the area between them with green color and transparency.

2. Step 2: Confirm fill_between works with any order

   fill_between fills area between y1 and y2 regardless of which is greater, so no error occurs.

3. Final Answer:

   A plot with two lines and a green shaded area between them -> Option D

4. Quick Check:

   Two lines + green fill_between area = A [OK]

Hint: fill_between shades area between lines regardless of order [OK]

Common Mistakes:

• Thinking fill_between needs y1 > y2 always
• Expecting no shaded area without fill_between call
• Confusing plot lines with fill_between shading

4. Identify the error in this code snippet that tries to fill between two time series:

import matplotlib.pyplot as plt
x = [0, 1, 2, 3]
y1 = [1, 2, 3]
y2 = [2, 3, 4, 5]
plt.fill_between(x, y1, y2, color='red')
plt.show()

medium

A. x and y1 have different lengths causing a ValueError

B. fill_between does not accept list inputs

C. Missing plt.plot calls before fill_between

D. Color argument must be uppercase

Solution

1. Step 1: Check lengths of x, y1, and y2

   x has 4 points, y1 has 3 points, y2 has 4 points; mismatch causes error.

2. Step 2: Understand fill_between input requirements

   fill_between requires x, y1, y2 arrays to have the same length to plot correctly.

3. Final Answer:

   x and y1 have different lengths causing a ValueError -> Option A

4. Quick Check:

   Length mismatch in inputs = B [OK]

Hint: Check all input arrays have same length before fill_between [OK]

Common Mistakes:

• Ignoring length mismatch errors
• Thinking fill_between needs plot calls first
• Assuming color argument case matters

5. You have two time series arrays y1 and y2 representing daily temperatures for two cities over 7 days. You want to highlight only the days when y1 is higher than y2 using fill_between. Which code snippet correctly achieves this?

hard

A. plt.fill_between(x, y1, y2, condition=(y1 > y2), color='orange')

B. plt.fill_between(x, y1, y2, where=(y1 > y2), color='orange', alpha=0.5)

C. plt.fill_between(x, y1, y2, where=(y1 < y2), color='orange', alpha=0.5)

D. plt.fill_between(x, y1, y2, color='orange', alpha=0.5)

Solution

1. Step 1: Use the 'where' parameter to specify condition

   The where argument in fill_between allows filling only where the condition is True.

2. Step 2: Set condition to y1 > y2 and specify color and transparency

   Using where=(y1 > y2) fills only days when y1 is higher than y2 with orange color and alpha 0.5.

3. Final Answer:

   plt.fill_between(x, y1, y2, where=(y1 > y2), color='orange', alpha=0.5) -> Option B

4. Quick Check:

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

Hint: Use 'where' to fill conditionally between lines [OK]

Common Mistakes:

• Using 'condition' instead of 'where'
• Filling without condition fills entire area
• Reversing the condition (y1 < y2) by mistake