Fig, ax = plt.subplots pattern in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

It returns two objects: fig, which is the Figure object representing the entire window or page, and ax, which is the Axes object where the actual plot is drawn.

beginner

Why do we use fig, ax = plt.subplots() instead of just plt.plot()?

Using fig, ax = plt.subplots() gives more control over the plot. You can customize the figure size, add multiple plots, and modify axes properties easily.

intermediate

How can you create multiple plots in one figure using plt.subplots()?

You can pass the number of rows and columns as arguments, like fig, axs = plt.subplots(2, 2). This creates a grid of 4 plots, and axs is an array of Axes objects.

beginner

What is the role of the fig object in the fig, ax = plt.subplots() pattern?

The fig object represents the whole figure or window. You can use it to set the figure size, save the figure to a file, or adjust layout settings.

beginner

How do you plot data on the axes object returned by plt.subplots()?

You call plotting methods on the ax object, like ax.plot(x, y). This draws the plot inside the axes area of the figure.

What does ax represent in fig, ax = plt.subplots()?

A The entire figure window

B A color map

C A data array

D The plotting area or axes

ax is the axes object where the plot is drawn, not the whole figure.

How do you create a 3x1 grid of plots using plt.subplots()?

A <code>fig, axs = plt.subplots(3)</code>

B <code>fig, ax = plt.subplots(1, 3)</code>

C <code>fig, axs = plt.subplots(3, 1)</code>

D <code>fig, ax = plt.subplots(1)</code>

Passing rows=3 and columns=1 creates a vertical stack of 3 plots.

Which object do you use to save the plot to a file?

A ax

B plt

C fig

D data

The fig object has the savefig() method to save the figure.

What is the default number of plots created by plt.subplots() with no arguments?

A 1

B 0

C 2

D 4

By default, it creates one figure and one axes (one plot).

How do you plot a line graph on the axes object?

A <code>ax.plot(x, y)</code>

B <code>fig.plot(x, y)</code>

C <code>plt.plot(x, y)</code>

D <code>ax.show(x, y)</code>

You use ax.plot(x, y) to plot on the specific axes.

Explain the purpose of the fig and ax objects returned by plt.subplots().

Think about the difference between the whole window and the area where you draw.

Describe how to create multiple plots in one figure using plt.subplots() and how to access each plot.

Consider a grid of small plots inside one big window.

Practice

1. What does the variable fig represent when you write fig, ax = plt.subplots() in matplotlib?

easy

A. The specific plot area where data is drawn

B. The whole figure container that holds all plot elements

C. A list of all plots in the figure

D. The title of the plot

Solution

1. Step 1: Understand the role of fig in plt.subplots()

   The fig variable holds the entire figure container, which can include multiple plots, titles, legends, and other elements.

2. Step 2: Differentiate fig from ax

   The ax variable is the specific plot area inside the figure where data is drawn, not the whole container.

3. Final Answer:

   The whole figure container that holds all plot elements -> Option B

4. Quick Check:

   fig = whole figure container [OK]

Hint: Remember: fig = full figure, ax = plot area [OK]

Common Mistakes:

• Confusing fig with ax as the plot area
• Thinking fig is just the plot title
• Assuming fig is a list of plots

2. Which of the following is the correct syntax to create a figure and axes using matplotlib's plt.subplots()?

easy

A. plt.subplots = fig, ax

B. fig = plt.subplots(ax)

C. ax, fig = plt.subplots()

D. fig, ax = plt.subplots()

Solution

1. Step 1: Recall the correct unpacking syntax

   The function plt.subplots() returns two objects: the figure and the axes. The correct way to capture them is fig, ax = plt.subplots().

2. Step 2: Check each option for syntax correctness

   fig = plt.subplots(ax) tries to pass ax as an argument incorrectly. ax, fig = plt.subplots() reverses the order, which is wrong. plt.subplots = fig, ax tries to assign incorrectly.

3. Final Answer:

   fig, ax = plt.subplots() -> Option D

4. Quick Check:

   Correct unpacking = fig, ax = plt.subplots() [OK]

Hint: Remember: fig first, then ax from plt.subplots() [OK]

Common Mistakes:

• Swapping fig and ax order
• Passing ax as argument to plt.subplots()
• Incorrect assignment syntax

3. What will be the output type of ax after running this code?

import matplotlib.pyplot as plt
fig, ax = plt.subplots()

medium

A.

B.

C.

D.

Solution

1. Step 1: Understand what plt.subplots() returns

   The function returns a figure object and an axes object. The axes object is usually an instance of AxesSubplot class.

2. Step 2: Identify the type of ax

   After unpacking, ax is the axes object, which is of type matplotlib.axes._subplots.AxesSubplot.

3. Final Answer:

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

4. Quick Check:

   ax type = AxesSubplot [OK]

Hint: ax is the plot area object, not the figure [OK]

Common Mistakes:

• Confusing ax type with fig type
• Thinking ax is a list or tuple
• Assuming ax is a string or number

4. Identify the error in this code snippet:

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

medium

A. fig and ax are reversed in assignment

B. ax.plot() arguments are incorrect

C. Missing parentheses after plt.subplots

D. plt.show() is called before plotting

Solution

1. Step 1: Check the call to plt.subplots

   The code uses plt.subplots without parentheses, so it assigns the function itself, not its result.

2. Step 2: Understand the impact on ax.plot

   Since fig, ax are assigned incorrectly, ax is a function, not an axes object, so ax.plot() will cause an error.

3. Final Answer:

   Missing parentheses after plt.subplots -> Option C

4. Quick Check:

   Call plt.subplots() with () [OK]

Hint: Always add () to call plt.subplots function [OK]

Common Mistakes:

• Forgetting parentheses on plt.subplots
• Mixing up fig and ax variables
• Calling plt.show() too early

5. You want to create a figure with 2 rows and 1 column of plots using plt.subplots(). Which code correctly creates the figure and axes, and plots a line on the second subplot?

hard

A. fig, ax = plt.subplots(2, 1)
ax[1].plot([1, 2, 3], [3, 2, 1])

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

C. fig, ax = plt.subplots(2, 1)
ax.plot([1, 2, 3], [3, 2, 1])

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

Solution

1. Step 1: Create figure with 2 rows and 1 column

   Use plt.subplots(2, 1) to create 2 vertical subplots. This returns ax as an array of axes.

2. Step 2: Plot on the second subplot

   Access the second subplot with ax[1] (indexing starts at 0) and call plot() on it.

3. Final Answer:

   fig, ax = plt.subplots(2, 1) ax[1].plot([1, 2, 3], [3, 2, 1]) -> Option A

4. Quick Check:

   2 rows, 1 col = plt.subplots(2,1), plot on ax[1] [OK]

Hint: Use ax[index] for multiple subplots, index starts at 0 [OK]

Common Mistakes:

• Using wrong subplot shape (1,2) instead of (2,1)
• Trying to plot on ax without indexing when multiple axes exist
• Using invalid index like ax[2] when only 2 subplots exist