Axes creation with add_subplot in Matplotlib - Mini Project: Build & Apply

Axes Creation with add_subplot in Matplotlib

📖 Scenario: You are working on a simple data visualization project. You want to create a plot with multiple sections (subplots) to compare different data sets side by side.

🎯 Goal: Build a Python script that creates a figure and adds two subplots using add_subplot. Each subplot will later hold a different plot.

📋 What You'll Learn

Create a figure object using plt.figure()

Add a first subplot to the figure using add_subplot(1, 2, 1)

Add a second subplot to the figure using add_subplot(1, 2, 2)

Print the axes objects to confirm creation

💡 Why This Matters

🌍 Real World

Creating multiple plots in one figure helps compare data sets side by side, useful in reports and presentations.

💼 Career

Data scientists and analysts often create multi-plot figures to visualize and compare different data trends clearly.

1

Create a figure object

Write code to create a figure object called fig using plt.figure().

Matplotlib

import matplotlib.pyplot as plt
# Create a figure object called fig
# Your code here

Hint

Use plt.figure() to create a new figure and assign it to fig.

2

Add the first subplot

Add a subplot to the figure fig using add_subplot(1, 2, 1) and assign it to a variable called ax1.

Matplotlib

import matplotlib.pyplot as plt
fig = plt.figure()
# Add the first subplot to fig and assign it to ax1
# Your code here

Hint

Use fig.add_subplot(1, 2, 1) to add the first subplot and save it as ax1.

3

Add the second subplot

Add a second subplot to the figure fig using add_subplot(1, 2, 2) and assign it to a variable called ax2.

Matplotlib

import matplotlib.pyplot as plt
fig = plt.figure()
ax1 = fig.add_subplot(1, 2, 1)
# Add the second subplot to fig and assign it to ax2
# Your code here

Hint

Use fig.add_subplot(1, 2, 2) to add the second subplot and save it as ax2.

4

Print the axes objects

Print the variables ax1 and ax2 to display the axes objects created by add_subplot.

Matplotlib

import matplotlib.pyplot as plt
fig = plt.figure()
ax1 = fig.add_subplot(1, 2, 1)
ax2 = fig.add_subplot(1, 2, 2)
# Print ax1 and ax2
# Your code here

Hint

Use print(ax1) and print(ax2) to show the axes objects.

Practice

1. What does the add_subplot method do in matplotlib?

easy

A. It removes a plot from the figure.

B. It saves the figure to a file.

C. It changes the color of the plot.

D. It adds a new plot (axes) to an existing figure.

Solution

1. Step 1: Understand the purpose of add_subplot

   The add_subplot method is used to add one or more plots (axes) inside a figure in matplotlib.

2. Step 2: Compare with other options

   Saving, coloring, or removing plots are done by other methods, not add_subplot.

3. Final Answer:

   It adds a new plot (axes) to an existing figure. -> Option D

4. Quick Check:

   add_subplot adds axes [OK]

Hint: Remember: add_subplot adds plots inside a figure [OK]

Common Mistakes:

• Confusing add_subplot with savefig
• Thinking it changes plot colors
• Assuming it deletes plots

2. Which of the following is the correct way to create a 2x2 grid of subplots and select the third subplot using add_subplot?

easy

A. fig.add_subplot(2, 2, 3)

B. fig.add_subplot(3, 2, 2)

C. fig.add_subplot(2, 3, 1)

D. fig.add_subplot(2, 2)

Solution

1. Step 1: Understand the parameters of add_subplot

   The method add_subplot(rows, columns, index) creates a grid and selects the subplot at the given index.

2. Step 2: Match the correct grid and index

   For a 2x2 grid, the third subplot is at position 3, so fig.add_subplot(2, 2, 3) is correct.

3. Final Answer:

   fig.add_subplot(2, 2, 3) -> Option A

4. Quick Check:

   2x2 grid, third subplot = add_subplot(2,2,3) [OK]

Hint: Use (rows, cols, position) to pick subplot [OK]

Common Mistakes:

• Mixing rows and columns order
• Omitting the position argument
• Using wrong grid size

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

import matplotlib.pyplot as plt
fig = plt.figure()
ax1 = fig.add_subplot(121)
ax2 = fig.add_subplot(122)
print(type(ax1), type(ax2))

medium

A.

B.

C.

D. SyntaxError

Solution

1. Step 1: Understand add_subplot with 3-digit code

   The code 121 means 1 row, 2 columns, subplot 1; 122 means subplot 2.

2. Step 2: Check the returned object types

   Both ax1 and ax2 are subplot axes objects of type AxesSubplot.

3. Final Answer:

   <class 'matplotlib.axes._subplots.AxesSubplot'> <class 'matplotlib.axes._subplots.AxesSubplot'> -> Option A

4. Quick Check:

   add_subplot returns AxesSubplot objects [OK]

Hint: 3-digit code means (rows, cols, index) [OK]

Common Mistakes:

• Expecting Figure type instead of Axes
• Confusing syntax with tuple input
• Thinking it causes syntax error

4. Identify the error in this code snippet:

import matplotlib.pyplot as plt
fig = plt.figure()
ax = fig.add_subplot(2, 2)
ax.plot([1, 2, 3], [4, 5, 6])
plt.show()

medium

A. plt.figure() should be plt.subplots() instead.

B. Missing the subplot position argument in add_subplot.

C. The plot data lists have different lengths.

D. plt.show() is called before plotting.

Solution

1. Step 1: Check add_subplot parameters

   The method requires three arguments: rows, columns, and position. Here, only two are given.

2. Step 2: Verify other parts of the code

   Plot data lists have equal length, plt.show() is correctly placed, and plt.figure() is valid.

3. Final Answer:

   Missing the subplot position argument in add_subplot. -> Option B

4. Quick Check:

   add_subplot needs 3 args (rows, cols, pos) [OK]

Hint: add_subplot needs three numbers: rows, cols, position [OK]

Common Mistakes:

• Forgetting the position argument
• Confusing plt.figure() with plt.subplots()
• Assuming plt.show() order matters here

5. You want to create a figure with 3 subplots arranged in 1 row and 3 columns. You want to add the second subplot and plot a line on it. Which code snippet correctly does this?

hard

A. fig = plt.figure()
ax = fig.add_subplot(3, 1, 2)
ax.plot([0, 1, 2], [2, 1, 0])
plt.show()

B. fig, ax = plt.subplots(3, 1)
ax[1].plot([0, 1, 2], [2, 1, 0])
plt.show()

C. fig = plt.figure()
ax = fig.add_subplot(1, 3, 2)
ax.plot([0, 1, 2], [2, 1, 0])
plt.show()

D. fig = plt.figure()
ax = fig.add_subplot(1, 2, 3)
ax.plot([0, 1, 2], [2, 1, 0])
plt.show()

Solution

1. Step 1: Understand subplot layout

   We want 1 row and 3 columns, so the call is add_subplot(1, 3, position).

2. Step 2: Select the second subplot and plot

   Position 2 selects the second subplot. The plot command is correct.

3. Step 3: Check other options

   fig, ax = plt.subplots(3, 1) ax[1].plot([0, 1, 2], [2, 1, 0]) plt.show() uses plt.subplots with wrong shape, C uses wrong layout, D uses invalid position 3 in 1x2 grid.

4. Final Answer:

   fig = plt.figure() ax = fig.add_subplot(1, 3, 2) ax.plot([0, 1, 2], [2, 1, 0]) plt.show() -> Option C

5. Quick Check:

   1 row, 3 cols, second subplot = add_subplot(1,3,2) [OK]

Hint: Match rows, cols, and position carefully [OK]

Common Mistakes:

• Mixing rows and columns numbers
• Using wrong subplot index
• Confusing plt.figure() and plt.subplots()