Time series with fill_between in Matplotlib - Mini Project: Build & Apply

Time Series Visualization with fill_between

📖 Scenario: You work as a data analyst for a weather station. You have daily temperature data for a week. You want to visualize the temperature changes and highlight the area between the minimum and maximum temperatures each day.

🎯 Goal: Create a time series plot of daily temperatures using matplotlib. Use fill_between to shade the area between the minimum and maximum temperatures for each day.

📋 What You'll Learn

Create lists for dates, minimum temperatures, and maximum temperatures.

Create a variable for the color to fill between the min and max temperatures.

Use plt.fill_between to shade the area between min and max temperatures.

Plot the min and max temperature lines on the same graph.

Display the plot with labels and a title.

💡 Why This Matters

🌍 Real World

Visualizing temperature ranges helps meteorologists and planners understand daily weather variations and prepare accordingly.

💼 Career

Data analysts and scientists often use time series plots with shaded areas to show ranges, confidence intervals, or variability in data.

1

Create the temperature data lists

Create three lists called dates, min_temps, and max_temps with these exact values:
dates = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
min_temps = [12, 14, 13, 15, 14, 16, 15]
max_temps = [20, 22, 21, 23, 22, 24, 23]

Matplotlib

# Create the lists for dates, min_temps, and max_temps
# Your code here

Hint

Use square brackets [] to create lists with the exact values given.

2

Set the fill color for the shaded area

Create a variable called fill_color and set it to the string 'lightblue' to use as the color for the area between min and max temperatures.

Matplotlib

dates = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
min_temps = [12, 14, 13, 15, 14, 16, 15]
max_temps = [20, 22, 21, 23, 22, 24, 23]
# Create a variable called fill_color and set it to 'lightblue'
# Your code here

Hint

Assign the string 'lightblue' to the variable fill_color.

3

Plot the temperature lines and fill the area between

Import matplotlib.pyplot as plt. Use plt.fill_between with dates, min_temps, and max_temps and the color set to fill_color. Then plot the min_temps and max_temps lines using plt.plot with dates as x-axis. Add labels for x-axis as 'Day' and y-axis as 'Temperature (°C)'. Add the title 'Daily Min and Max Temperatures'.

Matplotlib

dates = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
min_temps = [12, 14, 13, 15, 14, 16, 15]
max_temps = [20, 22, 21, 23, 22, 24, 23]
fill_color = 'lightblue'
# Import matplotlib.pyplot as plt
# Use plt.fill_between with dates, min_temps, max_temps, and color=fill_color
# Plot min_temps and max_temps lines with plt.plot
# Add x-axis label 'Day', y-axis label 'Temperature (°C)', and title 'Daily Min and Max Temperatures'
# Your code here

Hint

Remember to import matplotlib.pyplot as plt. Use plt.fill_between to shade between min and max temperatures. Then plot lines for min and max temperatures. Add labels and title for clarity.

4

Display the plot

Add a line to display the plot using plt.show().

Matplotlib

import matplotlib.pyplot as plt

dates = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
min_temps = [12, 14, 13, 15, 14, 16, 15]
max_temps = [20, 22, 21, 23, 22, 24, 23]
fill_color = 'lightblue'

plt.fill_between(dates, min_temps, max_temps, color=fill_color)
plt.plot(dates, min_temps, label='Min Temp', color='blue')
plt.plot(dates, max_temps, label='Max Temp', color='red')
plt.xlabel('Day')
plt.ylabel('Temperature (°C)')
plt.title('Daily Min and Max Temperatures')
plt.legend()
# Display the plot
# Your code here

Hint

Use plt.show() to display the plot window.

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