3D bar charts in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of 3D Bar Chart Data Arrays

What is the output of the following code snippet that prepares data for a 3D bar chart?

Matplotlib

import numpy as np
x = np.arange(3)
y = np.arange(2)
xpos, ypos = np.meshgrid(x, y)
xpos = xpos.flatten()
ypos = ypos.flatten()
zpos = np.zeros_like(xpos)
print(xpos, ypos, zpos)

A [0 1 2 0 1 2] [0 0 0 1 1 1] [0 0 0 0 0 0]

B [0 0 1 1 2 2] [0 1 0 1 0 1] [0 0 0 0 0 0]

C [0 1 2] [0 1] [0 0 0]

D [0 1 2 0 1 2] [0 1 0 1 0 1] [0 0 0 0 0 0]

💡 Hint

Think about how meshgrid arranges coordinates and what flatten() does.

np.meshgrid creates coordinate matrices from coordinate vectors. Flattening these matrices gives arrays of x and y positions for bars. zpos is zeros because bars start from zero height.

❓ data_output

intermediate

Number of Bars in 3D Bar Chart

Given x = range(4) and y = range(3), how many bars will be drawn in a 3D bar chart created by plotting all combinations of x and y?

A 7

B 9

C 6

D 12

💡 Hint

Multiply the number of x positions by the number of y positions.

Each x value pairs with each y value, so total bars = 4 * 3 = 12.

❓ visualization

advanced

Identify the Correct 3D Bar Chart Plot

Which option shows the correct 3D bar chart plot code that creates bars with heights from the list [1, 3, 2, 5] at positions (0,0), (1,0), (0,1), and (1,1)?

A ax.bar3d([0,1,0,1], [0,0,1,1], [0,0,0,0], 1, 1, [1,3,2,5])

B ax.bar3d([0,1,0,1], [0,0,1,1], [0,0,0,0], [1,1,1,1], [1,1,1,1], [1,3,2,5])

C ax.bar3d([0,1,0,1], [0,0,1,1], [1,3,2,5], 1, 1, 0)

D ax.bar3d([0,0,1,1], [0,1,0,1], [0,0,0,0], 1, 1, [1,3,2,5])

💡 Hint

Check the order and length of parameters for bar3d: xpos, ypos, zpos, dx, dy, dz.

bar3d requires arrays for x, y, z positions, widths dx, dy, and heights dz. Option B correctly provides all six parameters with matching lengths.

🔧 Debug

advanced

Error in 3D Bar Chart Plotting Code

What error will this code raise? import matplotlib.pyplot as plt from mpl_toolkits.mplot3d import Axes3D fig = plt.figure() ax = fig.add_subplot(111, projection='3d') x = [0,1] y = [0,1] z = [0,0] dx = dy = dz = [1,2] ax.bar3d(x, y, z, dx, dy, dz) plt.show()

Matplotlib

import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
x = [0,1]
y = [0,1]
z = [0,0]
dx = dy = dz = [1,2]
ax.bar3d(x, y, z, dx, dy, dz)
plt.show()

A TypeError: 'int' object is not iterable

B TypeError: bar3d() missing 3 required positional arguments

C ValueError: setting an array element with a sequence

D No error, plot displays correctly

💡 Hint

Check how dx, dy, dz are assigned and their types.

dx = dy = dz = [1,2] assigns the same list to all three variables. This causes bar3d to receive lists where it expects numbers or arrays of matching shape, leading to a ValueError.

🚀 Application

expert

Calculate Total Volume of Bars in 3D Bar Chart

Given the following data for a 3D bar chart: x = [0, 1, 2] y = [0, 1] dz = [2, 3, 1, 4, 2, 5] Each bar has width dx=1 and depth dy=1. What is the total volume of all bars combined?

A 17

B 18

C 19

D 20

💡 Hint

Volume of each bar = dx * dy * dz. Sum all volumes.

Each bar volume is 1 * 1 * dz value. Sum dz values: 2+3+1+4+2+5 = 17. Since dx and dy are 1, total volume = 17.

Practice

1. What does a 3D bar chart in matplotlib primarily represent?

easy

A. Data with two position dimensions and one height dimension

B. Only two-dimensional data with color coding

C. A line graph with three lines

D. A pie chart with depth effect

Solution

1. Step 1: Understand the axes in 3D bar charts

   3D bar charts use two axes for position (x and y) and one axis for height (z).

2. Step 2: Identify the data representation

   The height of each bar shows the value, while the base position shows categories or coordinates.

3. Final Answer:

   Data with two position dimensions and one height dimension -> Option A

4. Quick Check:

   3D bar chart = 2D position + height [OK]

Hint: Remember: 3D bars have x, y positions and z height [OK]

Common Mistakes:

• Confusing 3D bars with 2D bar charts
• Thinking 3D bars only show color differences
• Assuming 3D bars are line graphs

2. Which of the following is the correct way to create 3D axes in matplotlib before plotting a 3D bar chart?

easy

A. ax = plt.axes3d()

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

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

D. ax = plt.figure().add_subplot(111, projection='3d')

Solution

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

   The common method is to create a figure and add a 3D subplot using add_subplot(111, projection='3d').

2. Step 2: Check each option

   ax = plt.subplot(projection='3d') uses subplot instead of add_subplot, which is incorrect. ax = plt.subplots(projection='3d') returns a tuple (figure, axes), so assigning directly to ax is incorrect. ax = plt.axes3d() is not a valid matplotlib function.

3. Final Answer:

   ax = plt.figure().add_subplot(111, projection='3d') -> Option D

4. Quick Check:

   Use figure().add_subplot with projection='3d' [OK]

Hint: Use figure().add_subplot(111, projection='3d') to get 3D axes [OK]

Common Mistakes:

• Using plt.subplot instead of plt.figure().add_subplot
• Trying to call non-existent plt.axes3d()
• Confusing subplots() with subplot()

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

import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
x = [1, 2]
y = [3, 4]
z = [0, 0]
dx = dy = dz = [1, 1]
ax.bar3d(x, y, z, dx, dy, dz, color='red')
plt.show()

medium

A. Two red bars at positions (1,3) and (2,4) with height 1

B. Two red bars at positions (0,0) and (1,1) with height 1

C. Error because dx, dy, dz should be scalars, not lists

D. Empty plot with no bars

Solution

1. Step 1: Understand the parameters of ax.bar3d

   Parameters x, y, z are the positions of bars. dx, dy, dz are the sizes along each axis. Here, x=[1,2], y=[3,4], z=[0,0], and dx=dy=dz=[1,1].

2. Step 2: Analyze the plot output

   Two bars will appear at (1,3,0) and (2,4,0) with width=1, depth=1, height=1, colored red.

3. Final Answer:

   Two red bars at positions (1,3) and (2,4) with height 1 -> Option A

4. Quick Check:

   Positions and sizes match bars at (1,3) and (2,4) [OK]

Hint: Check x,y,z positions and dx,dy,dz sizes carefully [OK]

Common Mistakes:

• Assuming dx, dy, dz must be scalars only
• Confusing bar positions with sizes
• Expecting bars at (0,0) instead of given x,y

4. Identify the error in this code for plotting a 3D bar chart:

import matplotlib.pyplot as plt
fig = plt.figure()
ax = fig.add_subplot(111, projection='3d')
x = [1, 2, 3]
y = [4, 5]
z = [0, 0, 0]
dx = dy = dz = 1
ax.bar3d(x, y, z, dx, dy, dz)
plt.show()

medium

A. Missing import for Axes3D

B. Length of y does not match length of x and z

C. dx, dy, dz must be lists, not scalars

D. ax.bar3d does not accept scalar sizes

Solution

1. Step 1: Check lengths of position arrays

   x has length 3, y has length 2, z has length 3. They must all be the same length for bar3d.

2. Step 2: Verify size parameters

   dx, dy, dz can be scalars or lists matching length of bars, so scalars are allowed.

3. Final Answer:

   Length of y does not match length of x and z -> Option B

4. Quick Check:

   All position arrays must have equal length [OK]

Hint: Check all position lists have same length [OK]

Common Mistakes:

• Assuming dx, dy, dz must be lists
• Ignoring mismatch in array lengths
• Forgetting to import mpl_toolkits.mplot3d (not needed here)

5. You want to plot a 3D bar chart showing sales data for 3 products over 4 months. Which approach correctly sets up the data for ax.bar3d()?

hard

A. Use x, y, z all as sales values, dz as zeros

B. Use x as months, y as products, z as sales values, and dz as ones

C. Use x as product indices repeated for each month, y as month indices tiled for each product, z as zeros, and dz as sales values

D. Use x and y as sales values, z as product indices, dz as month indices

Solution

1. Step 1: Understand the data layout for 3D bars

   x and y represent positions (product and month), z is the base height (usually zero), dz is the height of bars (sales values).

2. Step 2: Arrange data correctly

   Repeat product indices for each month (x), tile month indices for each product (y), set z to zero, and use sales data as dz.

3. Final Answer:

   Use x as product indices repeated for each month, y as month indices tiled for each product, z as zeros, and dz as sales values -> Option C

4. Quick Check:

   Positions = product/month, height = sales [OK]

Hint: Map x,y to categories, dz to values for bar height [OK]

Common Mistakes:

• Mixing sales values as positions instead of heights
• Using z as sales height instead of dz
• Not repeating/tiling indices properly for grid