3D scatter plots in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a 3D scatter plot?

A 3D scatter plot is a graph that shows points in three dimensions (x, y, and z). It helps us see how three variables relate to each other in space.

beginner

Which matplotlib module is used to create 3D scatter plots?

The <code>mpl_toolkits.mplot3d</code> module provides 3D plotting tools, including the <code>Axes3D</code> class used to create 3D scatter plots.

beginner

How do you add a 3D scatter plot to a matplotlib figure?

First, create a 3D axes with fig.add_subplot(111, projection='3d'). Then use ax.scatter(x, y, z) to plot points in 3D.

intermediate

What parameters can you customize in a 3D scatter plot?

You can change point color, size, shape, and transparency. You can also label axes and add a title to make the plot clearer.

beginner

Why use 3D scatter plots instead of 2D scatter plots?

3D scatter plots show relationships among three variables at once, giving a fuller picture than 2D plots which show only two variables.

Which command creates a 3D axes in matplotlib?

A fig.add_subplot(111)

B plt.scatter3d()

C plt.plot3d()

D fig.add_subplot(111, projection='3d')

The correct way to create 3D axes is using fig.add_subplot(111, projection='3d'). Other options are not valid matplotlib commands.

What function plots points in a 3D scatter plot?

A ax.plot(x, y, z)

B plt.scatter(x, y)

C ax.scatter(x, y, z)

D plt.plot3d(x, y, z)

The ax.scatter(x, y, z) function plots points in 3D space on the 3D axes.

Which module must be imported to use 3D plotting in matplotlib?

A mpl_toolkits.mplot3d

B matplotlib.pyplot3d

C matplotlib3d

D mpl_3d

3D plotting tools are in the mpl_toolkits.mplot3d module.

What does the 'projection' parameter specify when adding a subplot?

A The type of plot, e.g., '3d' for 3D plots

B The color of the plot

C The size of the figure

D The data source

The 'projection' parameter tells matplotlib to create a 3D plot when set to '3d'.

Which of these can you NOT customize in a 3D scatter plot?

A Point color

B Background music

C Axis labels

D Point shape

Background music is not related to plotting and cannot be customized in matplotlib plots.

Explain how to create a basic 3D scatter plot using matplotlib.

Think about the steps from importing to displaying the plot.

Describe why 3D scatter plots are useful and when you might use them.

Consider the advantage of adding a third dimension to data visualization.

Practice

1. What is the main purpose of using a 3D scatter plot in matplotlib?

easy

A. To display text annotations in 3D space

B. To visualize data points in three dimensions and observe patterns

C. To plot a line graph with three lines

D. To create a bar chart with three bars

Solution

1. Step 1: Understand the role of 3D scatter plots

   3D scatter plots show points in three dimensions, helping to see relationships among three variables.

2. Step 2: Compare with other plot types

   Bar charts and line graphs do not show points in 3D space, and text annotations are not the main purpose.

3. Final Answer:

   To visualize data points in three dimensions and observe patterns -> Option B

4. Quick Check:

   3D scatter plots = visualize points in 3D [OK]

Hint: 3D scatter plots show points in 3D space [OK]

Common Mistakes:

• Confusing 3D scatter with bar or line plots
• Thinking 3D scatter is for text annotations
• Assuming 3D scatter plots show continuous surfaces

2. Which of the following is the correct way to create a 3D scatter plot axis in matplotlib?

easy

A. ax = plt.axes(projection='3d')

B. ax = plt.subplots(projection='3d')

C. ax = plt.figure(projection='3d')

D. ax = plt.subplot(projection='3d')

Solution

1. Step 1: Recall how to create 3D axes

   In matplotlib, plt.axes(projection='3d') creates a 3D axes object.

2. Step 2: Check other options

   plt.subplot and plt.subplots do not accept projection directly; plt.figure creates a figure, not axes.

3. Final Answer:

   ax = plt.axes(projection='3d') -> Option A

4. Quick Check:

   Use plt.axes with projection='3d' for 3D axes [OK]

Hint: Use plt.axes(projection='3d') to get 3D axes [OK]

Common Mistakes:

• Using plt.subplot instead of plt.axes for 3D
• Passing projection to plt.figure instead of axes
• Confusing plt.subplots with plt.subplot

3. What will be the output of this code snippet?

import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
ax.scatter([1, 2], [3, 4], [5, 6], c='r', marker='o')
plt.show()

medium

A. A 3D scatter plot with two red circular points at coordinates (1,3,5) and (2,4,6)

B. A 2D scatter plot with red points

C. A syntax error due to missing import

D. An empty plot with no points

Solution

1. Step 1: Analyze the code for 3D scatter plot creation

   The code creates a figure, adds a 3D subplot, and plots two points with coordinates (1,3,5) and (2,4,6) in red circles.

2. Step 2: Confirm the plot output

   The points will appear in 3D space as red circles; no errors occur.

3. Final Answer:

   A 3D scatter plot with two red circular points at coordinates (1,3,5) and (2,4,6) -> Option A

4. Quick Check:

   3D scatter with given points = red circles at (1,3,5) and (2,4,6) [OK]

Hint: Check coordinates and color for scatter points [OK]

Common Mistakes:

• Thinking it creates 2D plot instead of 3D
• Assuming syntax error without checking imports
• Expecting no points plotted

4. Identify the error in this code that tries to plot a 3D scatter plot:

import matplotlib.pyplot as plt
fig = plt.figure()
ax = fig.add_subplot(111)
ax.scatter([1,2,3], [4,5,6], [7,8,9])
plt.show()

medium

A. scatter function does not accept three arguments

B. The lists for x, y, z have different lengths

C. plt.figure() is not imported correctly

D. Missing projection='3d' in add_subplot, so 3D plotting fails

Solution

1. Step 1: Check subplot creation for 3D

   The code uses fig.add_subplot(111) without projection='3d', so it creates a 2D axes.

2. Step 2: Understand scatter with 3D data

   On 2D axes, passing three lists to scatter will treat the third list as point sizes instead of z-coordinates, producing a 2D scatter plot rather than 3D.

3. Final Answer:

   Missing projection='3d' in add_subplot, so 3D plotting fails -> Option D

4. Quick Check:

   3D scatter needs projection='3d' [OK]

Hint: Always add projection='3d' for 3D plots [OK]

Common Mistakes:

• Forgetting projection='3d' in add_subplot
• Thinking scatter can't take three arguments
• Assuming list length mismatch causes error

5. You want to plot a 3D scatter plot with points colored by their z-value using a colormap. Which code snippet correctly achieves this?

hard

A. ax.scatter(x, y, z, c='z', colormap='viridis')

B. ax.scatter(x, y, z, color='z', cmap='viridis')

C. ax.scatter(x, y, z, c=z, cmap='viridis')

D. ax.scatter(x, y, z, colors=z, cmap='viridis')

Solution

1. Step 1: Understand color mapping in scatter

   To color points by a variable, pass that variable to c= and specify cmap for colormap.

2. Step 2: Check correct parameter names

   c is correct for colors; color or colors with string 'z' or colormap are incorrect.

3. Final Answer:

   ax.scatter(x, y, z, c=z, cmap='viridis') -> Option C

4. Quick Check:

   Use c=variable and cmap='name' for color mapping [OK]

Hint: Use c=values and cmap='name' to color points [OK]

Common Mistakes:

• Using color='z' instead of c=z
• Using colormap instead of cmap
• Passing colors=z which is invalid