Area chart with plt.fill_between in Matplotlib - Mini Project: Build & Apply

Area chart with plt.fill_between

📖 Scenario: You are analyzing the sales data of a small shop over a week. You want to visualize how sales changed each day using an area chart. This will help you see the trend clearly.

🎯 Goal: Create an area chart using matplotlib with plt.fill_between to show daily sales over one week.

📋 What You'll Learn

Create a list called days with the days of the week as strings: 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'.

Create a list called sales with the sales numbers: 150, 200, 180, 220, 170, 210, 190.

Use plt.fill_between to create the area chart with days on the x-axis and sales on the y-axis.

Add labels for the x-axis and y-axis, and a title for the chart.

Display the chart using plt.show().

💡 Why This Matters

🌍 Real World

Area charts are useful to show how values change over time or categories, like sales, temperatures, or website visits.

💼 Career

Data analysts and scientists use area charts to communicate trends clearly to teams and decision makers.

1

Create the sales data lists

Create a list called days with these exact values: 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'. Then create a list called sales with these exact values: 150, 200, 180, 220, 170, 210, 190.

Matplotlib

# Create the lists days and sales with the exact values
# Your code here

Hint

Use square brackets to create lists. Put the days as strings in the days list and numbers in the sales list.

2

Import matplotlib and set up the plot

Import matplotlib.pyplot as plt. This will allow you to create the chart.

Matplotlib

days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
sales = [150, 200, 180, 220, 170, 210, 190]
# Import matplotlib.pyplot as plt
# Your code here

Hint

Use import matplotlib.pyplot as plt to import the plotting library.

3

Create the area chart with plt.fill_between

Use plt.fill_between with days as the x-axis and sales as the y-axis to create the area chart. Then add x-axis label 'Day', y-axis label 'Sales', and title 'Sales Over a Week'.

Matplotlib

import matplotlib.pyplot as plt

days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
sales = [150, 200, 180, 220, 170, 210, 190]
# Use plt.fill_between with days and sales
# Add labels and title
# Your code here

Hint

Use plt.fill_between with numeric x values and then set x-ticks to the day labels. Then use plt.xlabel, plt.ylabel, and plt.title to add labels and title.

4

Display the area chart

Use plt.show() to display the area chart you created.

Matplotlib

import matplotlib.pyplot as plt

days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
sales = [150, 200, 180, 220, 170, 210, 190]

plt.fill_between(range(len(days)), sales, color='skyblue', alpha=0.5)
plt.xticks(range(len(days)), days)
plt.xlabel('Day')
plt.ylabel('Sales')
plt.title('Sales Over a Week')
# Display the plot
# Your code here

Hint

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

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