Figure and axes creation in Data Analysis Python - Step-by-Step Execution

Concept Flow - Figure and axes creation

Import matplotlib.pyplot

↓

Create Figure object

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Add Axes or Subplots to Figure

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Use Axes to plot data

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Show or save the Figure

This flow shows how to import the plotting library, create a figure, add axes to it, plot data on the axes, and finally display or save the figure.

Execution Sample

Data Analysis Python

import matplotlib.pyplot as plt
fig = plt.figure()
ax = fig.add_subplot(111)
ax.plot([1, 2, 3], [4, 5, 6])
plt.show()

This code creates a figure, adds one axes, plots a simple line, and displays the plot window.

Execution Table

Step	Action	Object Created	State/Value	Output/Effect
1	Import matplotlib.pyplot as plt	plt module	Available for use	No visual output
2	Create figure with plt.figure()	Figure object	Empty figure with no axes	No visual output
3	Add subplot 111 to figure	Axes object	One axes added to figure	No visual output
4	Plot data on axes	Line2D object	Line plotted on axes	No visual output
5	Call plt.show()	N/A	Figure window opens	Plot window displays line graph
6	End of script	N/A	All objects remain in memory	Plot window remains open until closed

💡 Script ends after showing the plot window

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
plt	Not defined	matplotlib.pyplot module	matplotlib.pyplot module	matplotlib.pyplot module	matplotlib.pyplot module
fig	Not defined	Figure object (empty)	Figure object (with 1 axes)	Figure object (with 1 axes and line)	Figure object (with 1 axes and line)
ax	Not defined	Not defined	Axes object (subplot 111)	Axes object (with line plotted)	Axes object (with line plotted)

Key Moments - 3 Insights

Why do we need to create a Figure before adding Axes?

The Figure is the main container for plots. Without it, Axes have no place to live. See execution_table step 2 and 3 where Figure is created first, then Axes are added.

What does '111' mean in add_subplot(111)?

It means 1 row, 1 column, and this is the 1st subplot. This creates a single axes filling the figure. See execution_table step 3.

Why doesn't plotting data immediately show the plot?

Plotting adds data to the Axes but does not display it. plt.show() must be called to open the plot window. See execution_table step 4 vs step 5.

Visual Quiz - 3 Questions

Look at the execution table, what object is created at step 3?

A Figure object

B Line2D object

C Axes object

D plt module

💡 Hint

Check the 'Object Created' column at step 3 in the execution_table.

At which step does the plot window appear?

A Step 3

B Step 5

C Step 4

D Step 2

💡 Hint

Look at the 'Output/Effect' column to find when the figure window opens.

If we skip creating the Figure object, what happens when adding Axes?

A Error because no Figure exists

B Axes are added to a default figure automatically

C Axes are created but not shown

D Plot shows without axes

💡 Hint

The code calls fig.add_subplot(111), so 'fig' must be defined first; skipping causes NameError: name 'fig' is not defined.

Concept Snapshot

import matplotlib.pyplot as plt
fig = plt.figure()  # Create a figure container
ax = fig.add_subplot(111)  # Add one axes (1 row, 1 col, 1st plot)
ax.plot(x, y)  # Plot data on axes
plt.show()  # Display the figure window
Key: Figure holds Axes; Axes hold plots; show() displays all.

Full Transcript

This lesson shows how to create a figure and axes using matplotlib in Python. First, we import matplotlib.pyplot as plt. Then we create a Figure object with plt.figure(). Next, we add an Axes object to the figure using fig.add_subplot(111), which means one row, one column, first subplot. We plot data on the axes with ax.plot(). Finally, plt.show() opens a window displaying the plot. Variables plt, fig, and ax change as we create and add objects. Beginners often wonder why Figure must be created first, what '111' means, and why plotting doesn't immediately show the plot. The execution table traces each step and object creation. The visual quiz tests understanding of these steps. Remember: Figure is the container, Axes is the plot area, and show() displays the figure.

Practice

1. What does the plt.subplots() function return in matplotlib?

easy

A. A figure and one or more axes objects

B. Only a figure object

C. Only an axes object

D. A list of data points

Solution

1. Step 1: Understand plt.subplots() purpose

   The function is designed to create a figure and axes together for plotting.

2. Step 2: Check the return values

   It returns a tuple: the first is the figure, the second is the axes (single or array).

3. Final Answer:

   A figure and one or more axes objects -> Option A

4. Quick Check:

   plt.subplots() = (figure, axes) [OK]

Hint: Remember plt.subplots() returns figure and axes together [OK]

Common Mistakes:

• Thinking it returns only figure or only axes
• Confusing axes with data points
• Assuming it returns a plot directly

2. Which of the following is the correct syntax to create a figure with 2 rows and 3 columns of axes using matplotlib?

easy

A. fig, axes = plt.subplots(rows=3, cols=2)

B. fig, axes = plt.subplots(2, 3)

C. fig, axes = plt.subplots(3, 2)

D. fig, axes = plt.subplots(2*3)

Solution

1. Step 1: Recall plt.subplots() parameters

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

2. Step 2: Match the correct order

   To create 2 rows and 3 columns, use plt.subplots(2, 3).

3. Final Answer:

   fig, axes = plt.subplots(2, 3) -> Option B

4. Quick Check:

   Rows first, columns second in plt.subplots() [OK]

Hint: Remember rows first, columns second in plt.subplots() [OK]

Common Mistakes:

• Swapping rows and columns order
• Using keyword arguments incorrectly
• Passing a single number instead of two

3. What will be the shape of axes when running fig, axes = plt.subplots(2, 2)?

medium

A. A 2x2 numpy array of AxesSubplot objects

B. A list of 4 AxesSubplot objects

C. A single AxesSubplot object

D. A dictionary of axes objects

Solution

1. Step 1: Understand axes structure for multiple subplots

   When more than one subplot is created, axes is a 2D array matching the grid shape.

2. Step 2: Check the shape for 2 rows and 2 columns

   Axes will be a 2x2 numpy array of AxesSubplot objects.

3. Final Answer:

   A 2x2 numpy array of AxesSubplot objects -> Option A

4. Quick Check:

   Multiple subplots = axes array matching grid [OK]

Hint: Multiple subplots return axes array matching rows and columns [OK]

Common Mistakes:

• Expecting a flat list instead of 2D array
• Thinking axes is a single object always
• Confusing axes with figure

4. Identify the error in this code snippet:

fig, ax = plt.subplots(2, 2)
ax.plot([1, 2, 3], [4, 5, 6])

medium

A. fig and ax are swapped in assignment

B. plt.subplots() cannot create 2x2 axes

C. ax is an array, so ax.plot() causes an error

D. The plot data lists have different lengths

Solution

1. Step 1: Check the shape of ax from plt.subplots(2, 2)

   ax is a 2x2 array of axes, not a single axes object.

2. Step 2: Understand why ax.plot() fails

   Calling plot() on an array causes an AttributeError because arrays don't have plot method.

3. Final Answer:

   ax is an array, so ax.plot() causes an error -> Option C

4. Quick Check:

   Array of axes needs indexing before plot() [OK]

Hint: Index axes array before plotting when multiple subplots exist [OK]

Common Mistakes:

• Trying to plot on axes array directly
• Assuming plt.subplots(2,2) returns single axes
• Ignoring need to index axes for plotting

5. You want to create a figure with 3 subplots stacked vertically and set the figure size to 8 inches wide and 12 inches tall. Which code correctly does this?

hard

A. fig, axes = plt.subplots(1, 3, figsize=(12, 8))

B. fig, axes = plt.subplots(3, figsize=(8, 12))

C. fig, axes = plt.subplots(3, 1, size=(8, 12))

D. fig, axes = plt.subplots(3, 1, figsize=(8, 12))

Solution

1. Step 1: Determine subplot layout for vertical stacking

   3 subplots stacked vertically means 3 rows and 1 column.

2. Step 2: Set figure size correctly

   Use figsize=(width, height) with width=8 and height=12 inches.

3. Final Answer:

   fig, axes = plt.subplots(3, 1, figsize=(8, 12)) -> Option D

4. Quick Check:

   Rows=3, cols=1, figsize=(8,12) [OK]

Hint: Use figsize=(width, height) and rows, columns order [OK]

Common Mistakes:

• Swapping rows and columns
• Using wrong keyword 'size' instead of 'figsize'
• Confusing width and height order