How Matplotlib renders figures

Introduction

Matplotlib draws pictures (figures) so you can see your data as graphs. Understanding how it creates these pictures helps you make better charts.

You want to show data trends with a line or bar chart.

You need to save a graph as an image file to share.

You want to customize how your chart looks before showing it.

You want to update a chart dynamically in a program.

You want to understand why your chart looks a certain way.

Syntax

Matplotlib

import matplotlib.pyplot as plt

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

fig is the whole picture (figure).

ax is the area where the data is drawn (axes).

Examples

This creates a figure and adds one plot area manually.

Matplotlib

import matplotlib.pyplot as plt

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

This creates two stacked plots in one figure.

Matplotlib

import matplotlib.pyplot as plt

fig, ax = plt.subplots(2, 1)
ax[0].plot([1, 2, 3], [4, 5, 6])
ax[1].bar([1, 2, 3], [6, 5, 4])
plt.show()

Sample Program

This code creates a figure with one plot area, draws a line graph, adds labels and a title, then shows the figure window.

Matplotlib

import matplotlib.pyplot as plt

# Create a figure and axes
fig, ax = plt.subplots()

# Plot some data
ax.plot([0, 1, 2, 3], [10, 20, 25, 30], label='Line')

# Add title and labels
ax.set_title('Simple Line Plot')
ax.set_xlabel('X axis')
ax.set_ylabel('Y axis')

# Show legend
ax.legend()

# Display the figure
plt.show()

Important Notes

Matplotlib first creates a Figure object which is like a blank canvas.

Inside the figure, Axes objects are created where data is drawn.

When you call plt.show(), Matplotlib renders the figure on your screen.

Summary

Matplotlib draws figures by creating a Figure and one or more Axes inside it.

You add data to Axes, then call plt.show() to see the graph.

Understanding this helps you control how your charts look and behave.

Practice

1. What is the first step Matplotlib takes when creating a plot?

easy

A. Add data points directly to the screen

B. Display the plot with plt.show()

C. Save the plot to a file automatically

D. Create a figure and axes to draw on

Solution

1. Step 1: Understand Matplotlib's drawing process

   Matplotlib starts by creating a figure and axes, which are the canvas and area to draw on.

2. Step 2: Identify the first action in plotting

   Before adding data or showing the plot, the figure and axes must exist to hold the plot.

3. Final Answer:

   Create a figure and axes to draw on -> Option D

4. Quick Check:

   First step = create figure and axes [OK]

Hint: Remember: figure and axes come before plotting data [OK]

Common Mistakes:

• Thinking plt.show() creates the plot
• Assuming data is drawn directly without axes
• Believing plot saves automatically without show

2. Which of the following is the correct way to display a Matplotlib figure?

easy

A. plt.show()

B. plt.draw()

C. plt.display()

D. plt.render()

Solution

1. Step 1: Recall the function to display plots

   Matplotlib uses plt.show() to open a window and display the figure.

2. Step 2: Check other options for correctness

   plt.draw() updates the figure but does not display it; plt.display() and plt.render() do not exist in Matplotlib.

3. Final Answer:

   plt.show() -> Option A

4. Quick Check:

   Display function = plt.show() [OK]

Hint: Use plt.show() to see your plot on screen [OK]

Common Mistakes:

• Using plt.draw() instead of plt.show()
• Trying plt.display() which is not a Matplotlib function
• Confusing plt.render() with other libraries

3. What will the following code display?

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

medium

A. A figure with a line connecting points (1,4), (2,5), (3,6)

B. A blank figure with no lines

C. An error because plt.subplots() is incorrect

D. A scatter plot of points (1,4), (2,5), (3,6)

Solution

1. Step 1: Understand the code's plotting commands

   plt.subplots() creates figure and axes; ax.plot() draws a line connecting given points.

2. Step 2: Identify the plot type and output

   ax.plot() connects points with lines, so the figure shows a line through (1,4), (2,5), (3,6).

3. Final Answer:

   A figure with a line connecting points (1,4), (2,5), (3,6) -> Option A

4. Quick Check:

   ax.plot() draws lines through points [OK]

Hint: ax.plot() draws lines; scatter needs ax.scatter() [OK]

Common Mistakes:

• Thinking ax.plot() creates scatter points only
• Assuming plt.subplots() causes an error
• Expecting no lines without plt.show()

4. What is wrong with this code snippet?

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

medium

A. plt.plot() cannot be used without axes

B. fig.show() does not display the plot; plt.show() is needed

C. plt.figure() must be called after plt.plot()

D. The lists have different lengths causing an error

Solution

1. Step 1: Analyze the display method used

   fig.show() is not the standard way to display plots; plt.show() is required to render the figure window.

2. Step 2: Check other parts for errors

   plt.plot() works without explicit axes; lists have equal length; plt.figure() order is correct.

3. Final Answer:

   fig.show() does not display the plot; plt.show() is needed -> Option B

4. Quick Check:

   Use plt.show() to display figures [OK]

Hint: Always end with plt.show() to display plots [OK]

Common Mistakes:

• Using fig.show() instead of plt.show()
• Thinking plt.plot() needs axes object
• Assuming list length mismatch causes error here

5. You want to create a figure with two side-by-side plots sharing the same y-axis. Which code correctly sets this up before plotting data?

hard

A. fig, (ax1, ax2) = plt.subplots(2, 2, sharey=True)

B. fig, ax = plt.subplots(2, 1, sharex=True)

C. fig, (ax1, ax2) = plt.subplots(1, 2, sharey=True)

D. fig = plt.figure()
ax1 = fig.add_subplot(121)
ax2 = fig.add_subplot(122)

Solution

1. Step 1: Understand subplot layout for side-by-side plots

   Side-by-side means 1 row and 2 columns, so plt.subplots(1, 2) is correct.

2. Step 2: Check sharing y-axis option

   sharey=True shares the y-axis between the two plots as required.

3. Step 3: Verify other options

   fig, ax = plt.subplots(2, 1, sharex=True) creates vertical plots sharing x-axis; fig = plt.figure() ax1 = fig.add_subplot(121) ax2 = fig.add_subplot(122) creates subplots but does not share y-axis; fig, (ax1, ax2) = plt.subplots(2, 2, sharey=True) creates 4 plots (2x2), not 2 side-by-side.

4. Final Answer:

   fig, (ax1, ax2) = plt.subplots(1, 2, sharey=True) -> Option C

5. Quick Check:

   Side-by-side = (1,2), share y-axis = sharey=True [OK]

Hint: Use plt.subplots(1, 2, sharey=True) for side-by-side shared y-axis [OK]

Common Mistakes:

• Confusing rows and columns in plt.subplots()
• Forgetting to set sharey=True for shared y-axis
• Using add_subplot without sharing axes