Inverted axes in Matplotlib - Mini Project: Build & Apply

Inverted axes

📖 Scenario: You are analyzing sales data for a small store. You want to visualize the sales over a week using a bar chart. To better understand the data, you want to invert the axes so that the days appear on the vertical axis and the sales values on the horizontal axis.

🎯 Goal: Create a bar chart using matplotlib with inverted axes. The days of the week should be on the vertical axis, and the sales numbers on the horizontal axis.

📋 What You'll Learn

Create a dictionary called sales with days of the week as keys and sales numbers as values.

Create a list called days containing the days of the week in order.

Create a list called values containing the sales numbers in the same order as days.

Use matplotlib.pyplot to create a bar chart with inverted axes.

Invert the axes so that days appear on the vertical axis and sales on the horizontal axis.

Display the plot.

💡 Why This Matters

🌍 Real World

Inverting axes is useful when you want to display categories on the vertical axis for better readability, such as days of the week or product names.

💼 Career

Data analysts and scientists often customize plots to make data easier to understand and communicate insights clearly to others.

1

Create the sales data dictionary

Create a dictionary called sales with these exact entries: 'Monday': 150, 'Tuesday': 200, 'Wednesday': 170, 'Thursday': 220, 'Friday': 180.

Matplotlib

# Create the sales dictionary with days and sales values
# Your code here

Hint

Use curly braces {} to create the dictionary with the exact keys and values.

2

Create lists for days and sales values

Create a list called days containing 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday' in this exact order. Then create a list called values containing the sales numbers from the sales dictionary in the same order as days.

Matplotlib

sales = {'Monday': 150, 'Tuesday': 200, 'Wednesday': 170, 'Thursday': 220, 'Friday': 180}
# Create the list days with the days of the week
# Create the list values with sales numbers in the same order
# Your code here

Hint

Use a list comprehension to get the sales values in the order of the days list.

3

Create the bar chart with inverted axes

Import matplotlib.pyplot as plt. Use plt.barh(days, values) to create a horizontal bar chart with days on the vertical axis and values on the horizontal axis.

Matplotlib

sales = {'Monday': 150, 'Tuesday': 200, 'Wednesday': 170, 'Thursday': 220, 'Friday': 180}
days = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday']
values = [sales[day] for day in days]
# Import matplotlib.pyplot as plt
# Create a horizontal bar chart with plt.barh(days, values)
# Your code here

Hint

Use plt.barh() to create a horizontal bar chart which inverts the axes.

4

Display the plot

Use plt.show() to display the bar chart with inverted axes.

Matplotlib

import matplotlib.pyplot as plt

sales = {'Monday': 150, 'Tuesday': 200, 'Wednesday': 170, 'Thursday': 220, 'Friday': 180}
days = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday']
values = [sales[day] for day in days]

plt.barh(days, values)
# Display the plot
# Your code here

Hint

Call plt.show() to open the window with the plot.

Practice

1. What does the invert_xaxis() method do in a matplotlib plot?

easy

A. It changes the x-axis scale from linear to logarithmic.

B. It swaps the x-axis with the y-axis.

C. It hides the x-axis labels and ticks.

D. It reverses the direction of the x-axis, so values decrease from left to right.

Solution

1. Step 1: Understand the purpose of invert_xaxis()

   The method invert_xaxis() reverses the x-axis direction, making values decrease from left to right instead of increasing.

2. Step 2: Compare with other options

   Options A, C, and D describe different actions unrelated to axis inversion.

3. Final Answer:

   It reverses the direction of the x-axis, so values decrease from left to right. -> Option D

4. Quick Check:

   invert_xaxis() reverses x-axis direction [OK]

Hint: invert_xaxis() flips x-axis direction left to right [OK]

Common Mistakes:

• Confusing invert_xaxis() with changing scale
• Thinking it hides axis labels
• Assuming it swaps axes

2. Which of the following is the correct way to invert the y-axis in matplotlib?

easy

A. plt.invert_yaxis()

B. ax.axis.invert_y()

C. ax.invert_yaxis()

D. plt.axis('invert_y')

Solution

1. Step 1: Recall the correct syntax for axis inversion

   In matplotlib, axis inversion methods are called on the axis object, not directly on plt.

2. Step 2: Identify the correct method call

   The correct method is invert_yaxis() called on an axis object, typically named ax.

3. Final Answer:

   ax.invert_yaxis() -> Option C

4. Quick Check:

   Use ax.invert_yaxis() to invert y-axis [OK]

Hint: Call invert_yaxis() on axis object, not plt [OK]

Common Mistakes:

• Calling invert_yaxis() on plt instead of ax
• Using incorrect method names
• Trying to invert axis via plt.axis()

3. What will be the effect of the following code snippet?

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot([1, 2, 3], [10, 20, 30])
ax.invert_xaxis()
plt.show()

medium

A. The x-axis will display values from 3 to 1 left to right.

B. The x-axis will display values from 1 to 3 left to right.

C. The y-axis will be inverted, showing 30 to 10 bottom to top.

D. The plot will raise an error because invert_xaxis() is invalid.

Solution

1. Step 1: Analyze the code's axis inversion

   The code calls ax.invert_xaxis(), which reverses the x-axis direction.

2. Step 2: Understand axis direction after inversion

   Normally, x-axis goes from 1 to 3 left to right; after inversion, it goes from 3 to 1 left to right.

3. Final Answer:

   The x-axis will display values from 3 to 1 left to right. -> Option A

4. Quick Check:

   invert_xaxis() reverses x-axis values [OK]

Hint: invert_xaxis() flips x-axis values order [OK]

Common Mistakes:

• Thinking invert_xaxis() affects y-axis
• Assuming axis direction stays the same
• Believing invert_xaxis() causes error

4. Identify the error in this code that tries to invert the y-axis:

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
ax.plot([1, 2, 3], [10, 20, 30])
plt.invert_yaxis()
plt.show()

medium

A. invert_yaxis() should be called on ax, not plt.

B. invert_yaxis() does not exist in matplotlib.

C. The plot function is missing axis inversion parameter.

D. plt.show() must be called before invert_yaxis().

Solution

1. Step 1: Check where invert_yaxis() is called

   The code calls invert_yaxis() on plt, which is incorrect.

2. Step 2: Correct usage of invert_yaxis()

   This method must be called on the axis object ax, not on the pyplot module plt.

3. Final Answer:

   invert_yaxis() should be called on ax, not plt. -> Option A

4. Quick Check:

   Call invert_yaxis() on ax object [OK]

Hint: Call invert_yaxis() on ax, not plt [OK]

Common Mistakes:

• Calling invert_yaxis() on plt instead of ax
• Assuming invert_yaxis() is a plt method
• Calling plt.show() before inversion

5. You want to plot temperature decreasing over time, but your data has time increasing left to right. How can you use inverted axes to show time decreasing from left to right and temperature increasing bottom to top? Choose the correct code snippet.

hard

A. ax.plot(time, temperature)
ax.invert_xaxis()
ax.invert_yaxis()

B. ax.plot(time, temperature)
ax.invert_xaxis()

C. ax.plot(time, temperature)
ax.invert_yaxis()

D. ax.plot(time, temperature)

Solution

1. Step 1: Understand the desired axis directions

   Time should decrease from left to right, so x-axis must be inverted. Temperature should increase bottom to top, so y-axis stays normal.

2. Step 2: Choose the correct inversion method

   Only ax.invert_xaxis() is needed to flip time axis; inverting y-axis would reverse temperature direction incorrectly.

3. Final Answer:

   ax.plot(time, temperature) ax.invert_xaxis() -> Option B

4. Quick Check:

   Invert x-axis only to reverse time direction [OK]

Hint: Invert only the axis that needs reversed direction [OK]

Common Mistakes:

• Inverting both axes unnecessarily
• Not inverting any axis
• Inverting y-axis instead of x-axis