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.