Axes creation with add_subplot in Matplotlib

Introduction

We use add_subplot to add one or more plots inside a figure. It helps organize multiple graphs in rows and columns.

When you want to show multiple charts in one window.

When comparing different data sets side by side.

When you want to create a grid of plots for better visualization.

When you want to add a single plot to an existing figure.

When you want to control the position of each plot inside the figure.

Syntax

Matplotlib

ax = fig.add_subplot(nrows, ncols, index)

# or using a 3-digit code
ax = fig.add_subplot(abc)  # where a=nrows, b=ncols, c=index

nrows and ncols define the grid size.

index is the position of the subplot (starts at 1).

Examples

This creates a 2x2 grid and adds two subplots in the first and second positions.

Matplotlib

fig = plt.figure()
ax1 = fig.add_subplot(2, 2, 1)  # top-left plot
ax2 = fig.add_subplot(2, 2, 2)  # top-right plot

Using the 3-digit code 111 means 1 row, 1 column, 1st plot (just one plot).

Matplotlib

fig = plt.figure()
ax = fig.add_subplot(111)  # single plot in 1x1 grid

This adds a subplot in the middle of a vertical stack of 3 plots.

Matplotlib

fig = plt.figure()
ax = fig.add_subplot(3, 1, 2)  # middle plot in 3 rows, 1 column

Sample Program

This code creates a figure with 4 plots arranged in 2 rows and 2 columns. Each plot shows a simple line graph with a title.

Matplotlib

import matplotlib.pyplot as plt

fig = plt.figure(figsize=(6, 4))

# Create a 2x2 grid of plots
ax1 = fig.add_subplot(2, 2, 1)
ax2 = fig.add_subplot(2, 2, 2)
ax3 = fig.add_subplot(2, 2, 3)
ax4 = fig.add_subplot(2, 2, 4)

# Plot simple lines in each subplot
ax1.plot([1, 2, 3], [1, 4, 9])
ax1.set_title('Plot 1')

ax2.plot([1, 2, 3], [2, 3, 4])
ax2.set_title('Plot 2')

ax3.plot([1, 2, 3], [5, 2, 1])
ax3.set_title('Plot 3')

ax4.plot([1, 2, 3], [3, 6, 9])
ax4.set_title('Plot 4')

plt.tight_layout()
plt.show()

Important Notes

The index in add_subplot starts at 1, not 0.

You can use plt.subplots() for a simpler way to create multiple axes, but add_subplot gives more control.

Remember to call plt.show() to display the figure.

Summary

add_subplot helps add one or more plots inside a figure in a grid layout.

You specify the grid size and position to control where each plot appears.

This is useful for comparing multiple charts in one window.

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()