Plt.subplots with rows and columns in Matplotlib - Step-by-Step Execution

Concept Flow - Plt.subplots with rows and columns

Call plt.subplots(rows, cols)

↓

Create Figure object

↓

Create Axes array with shape (rows, cols)

↓

Return Figure and Axes

↓

User plots on each Axes

↓

Display or save the figure

The function plt.subplots creates a figure and a grid of subplots (axes) arranged in rows and columns, returning them for plotting.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
fig, axs = plt.subplots(2, 3)
axs[0, 0].plot([1, 2], [3, 4])
plt.show()

This code creates a 2-row by 3-column grid of plots and draws a line on the first subplot.

Execution Table

Step	Action	Result	Notes
1	Call plt.subplots(2, 3)	Figure and 2x3 Axes array created	Figure is empty, axes ready for plotting
2	Access axs[0, 0]	First subplot Axes object	Top-left plot in grid
3	Plot line on axs[0, 0]	Line appears on first subplot	Data points (1,3) and (2,4) connected
4	Call plt.show()	Figure window opens with 6 subplots	Only first subplot has a line, others are empty
5	End	Execution stops	All plots displayed

💡 All subplots created and displayed, program ends after plt.show()

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
fig	None	Figure object	Figure object	Figure object
axs	None	2x3 numpy array of Axes	2x3 numpy array of Axes with line on axs[0,0]	Same as after step 3

Key Moments - 3 Insights

Why is axs a 2D array and not a list?

Because plt.subplots with rows and columns returns a numpy array shaped (rows, cols), so axs[0,0] accesses the first subplot. See execution_table step 2.

What happens if you try axs[3,0] in this example?

It causes an IndexError because axs only has 2 rows (indices 0 and 1). See variable_tracker for axs shape.

Why do only some subplots show data after plotting?

Because we only plotted on axs[0,0]. The other subplots are empty by default. See execution_table step 3 and 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the shape of axs after step 1?

A 1D array with 6 Axes

B List of 2 Axes

C 2D array with shape (2, 3)

D Single Axes object

💡 Hint

Check execution_table row 1 under Result describing axs shape

At which step does the first line appear on the plot?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

See execution_table step 3 where plotting happens

If you change plt.subplots(2, 3) to plt.subplots(3, 2), how does axs shape change?

A axs becomes 1D array

B Shape becomes (3, 2)

C Shape stays (2, 3)

D axs becomes a list

💡 Hint

The rows and columns arguments define axs shape as (rows, cols)

Concept Snapshot

plt.subplots(rows, cols) creates a Figure and a grid of Axes with given rows and columns.
Returns (fig, axs) where axs is a 2D array of Axes.
Access subplots by axs[row, col].
Plot on each Axes independently.
Call plt.show() to display all subplots.

Full Transcript

This visual execution trace shows how plt.subplots with rows and columns works. First, plt.subplots(2, 3) creates a figure and a 2 by 3 grid of subplot axes. The axs variable holds these axes in a 2D array. We then plot a line on the first subplot axs[0, 0]. Calling plt.show() opens a window displaying all six subplots, with only the first showing the line. The variable tracker confirms fig and axs states after each step. Key moments clarify common confusions about axs shape and indexing. The quiz tests understanding of axs shape, plotting step, and effect of changing rows and columns. The snapshot summarizes the main points for quick reference.

Practice

1. What does plt.subplots(2, 3) create in matplotlib?

easy

A. A figure with 2 rows and 3 columns of subplots

B. A figure with 3 rows and 2 columns of subplots

C. A single plot with size 2x3 inches

D. A figure with 6 rows and 1 column of subplots

Solution

1. Step 1: Understand plt.subplots parameters

   The first parameter is the number of rows, the second is columns.

2. Step 2: Apply to given code

   plt.subplots(2, 3) means 2 rows and 3 columns of subplots.

3. Final Answer:

   A figure with 2 rows and 3 columns of subplots -> Option A

4. Quick Check:

   rows=2, columns=3 = B [OK]

Hint: First number = rows, second = columns in plt.subplots [OK]

Common Mistakes:

• Confusing rows and columns order
• Thinking parameters set figure size
• Assuming single plot created

2. Which of the following is the correct syntax to create a 3x1 subplot grid using plt.subplots?

easy

A. fig, axes = plt.subplots(1, 3)

B. fig, axes = plt.subplots(3)

C. fig, axes = plt.subplots(3, 1)

D. fig, axes = plt.subplots(rows=3, cols=1)

Solution

1. Step 1: Recall plt.subplots parameter order

   The first argument is rows, second is columns.

2. Step 2: Match to 3 rows and 1 column

   plt.subplots(3, 1) creates 3 rows and 1 column.

3. Final Answer:

   fig, axes = plt.subplots(3, 1) -> Option C

4. Quick Check:

   3 rows, 1 column = C [OK]

Hint: Remember: plt.subplots(rows, columns) order matters [OK]

Common Mistakes:

• Swapping rows and columns
• Using single number for both dimensions
• Using keyword arguments incorrectly

3. What is the shape of axes returned by fig, axes = plt.subplots(2, 2)?

medium

A. A 1D array with 4 axes objects

B. A 2D array with shape (2, 2) of axes objects

C. A list of 2 axes objects

D. A single axes object

Solution

1. Step 1: Understand axes shape for multiple rows and columns

   When rows and columns are more than 1, axes is a 2D array matching shape.

2. Step 2: Apply to plt.subplots(2, 2)

   axes will be a 2x2 array of axes objects.

3. Final Answer:

   A 2D array with shape (2, 2) of axes objects -> Option B

4. Quick Check:

   Multiple rows and columns = 2D axes array [OK]

Hint: Multiple rows & columns -> axes is 2D array [OK]

Common Mistakes:

• Assuming axes is always 1D
• Confusing number of axes with shape
• Expecting a single axes object

4. Given the code:

fig, axes = plt.subplots(2, 2)
axes[2, 0].plot([1, 2, 3])

What error will occur?

medium

A. No error, plot will display

B. TypeError: axes object is not subscriptable

C. AttributeError: 'AxesSubplot' object has no attribute 'plot'

D. IndexError: index 2 is out of bounds for axis 0 with size 2

Solution

1. Step 1: Check axes shape from plt.subplots(2, 2)

   axes is a 2x2 array with indices 0 and 1 for rows and columns.

2. Step 2: Analyze axes[2, 0] access

   Index 2 for rows is out of bounds since max index is 1.

3. Final Answer:

   IndexError: index 2 is out of bounds for axis 0 with size 2 -> Option D

4. Quick Check:

   axes shape (2,2), index 2 invalid [OK]

Hint: Check axes array shape before indexing [OK]

Common Mistakes:

• Using invalid indices beyond shape
• Confusing axes as list instead of array
• Expecting no error on wrong index

5. You want to create a 2x3 grid of subplots and plot a line on each subplot with data from a list of 6 arrays. Which code correctly accesses each subplot to plot inside a nested loop?

hard

A. for i in range(2):
for j in range(3):
axes[i][j].plot(data[i*3 + j])

B. for i in range(2):
for j in range(3):
axes[i, j].plot(data[i + j])

C. for i in range(3):
for j in range(2):
axes[i][j].plot(data[i*2 + j])

D. for i in range(2):
for j in range(3):
axes[j][i].plot(data[i*3 + j])

Solution

1. Step 1: Understand axes shape and indexing

   plt.subplots(2, 3) creates axes as a 2x3 array, indexed as axes[row][col].

2. Step 2: Correctly map data list to subplot indices

   Data index = row * number_of_columns + column = i*3 + j matches subplot position.

3. Final Answer:

   for i in range(2): for j in range(3): axes[i][j].plot(data[i*3 + j]) -> Option A

4. Quick Check:

   axes[row][col], data index = row*cols + col [OK]

Hint: Use axes[row][col] and data index = row*cols + col [OK]

Common Mistakes:

• Swapping row and column loops
• Incorrect data indexing
• Using axes[j][i] instead of axes[i][j]