Bar chart color customization in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Bar chart with single color

What color will the bars be in this matplotlib bar chart code?

Matplotlib

import matplotlib.pyplot as plt

values = [3, 7, 5]
labels = ['A', 'B', 'C']
plt.bar(labels, values, color='green')
plt.show()

A Blue bars

B Green bars

C Red bars

D Bars with default color (blue)

💡 Hint

Look at the color argument in plt.bar function.

The color parameter is set to 'green', so all bars will be green.

❓ data_output

intermediate

Bar chart with list of colors

What colors will the bars have in this matplotlib code?

Matplotlib

import matplotlib.pyplot as plt

values = [4, 6, 8]
labels = ['X', 'Y', 'Z']
colors = ['red', 'blue', 'yellow']
plt.bar(labels, values, color=colors)
plt.show()

A Bars colored red, blue, yellow respectively

B All bars red

C Bars colored blue, red, yellow respectively

D Bars colored yellow, blue, red respectively

💡 Hint

The color argument accepts a list matching bars order.

Each bar gets the color from the list in order: first red, then blue, then yellow.

❓ visualization

advanced

Custom color for bars based on value

Which option produces a bar chart where bars with values above 5 are red, others are green?

Matplotlib

import matplotlib.pyplot as plt

values = [2, 7, 4, 9]
labels = ['P', 'Q', 'R', 'S']
colors = ['red' if v > 5 else 'green' for v in values]
plt.bar(labels, values, color=colors)
plt.show()

A Bars with values > 5 are green, others red

B All bars are green

C Bars alternate red and green regardless of value

D Bars with values > 5 are red, others green

💡 Hint

Look at the list comprehension for colors.

The list comprehension sets 'red' for values above 5, else 'green'.

🔧 Debug

advanced

Identify error in bar color assignment

What error occurs when running this code?

Matplotlib

import matplotlib.pyplot as plt

values = [1, 2, 3]
labels = ['a', 'b', 'c']
colors = 'red, blue, green'
plt.bar(labels, values, color=colors)
plt.show()

A TypeError because color expects a list, not a string

B SyntaxError due to missing brackets

C No error, bars colored red, blue, green

D ValueError because colors list length mismatch

💡 Hint

Check the type of colors variable.

colors is a single string, not a list, so matplotlib cannot assign colors per bar.

🚀 Application

expert

Create a bar chart with gradient colors

Which code snippet correctly creates a bar chart with bars colored from light blue to dark blue using matplotlib?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

values = [5, 10, 15, 20]
labels = ['W', 'X', 'Y', 'Z']
# Fill in the color assignment
colors = [plt.cm.Blues(i) for i in np.linspace(0.3, 1, len(values))]
plt.bar(labels, values, color=colors)
plt.show()

A colors = plt.cm.Blues(np.linspace(0.3, 1, len(values)))

B colors = ['lightblue', 'blue', 'darkblue', 'navy']

C colors = [plt.cm.Blues(i) for i in np.linspace(0.3, 1, len(values))]

D colors = [plt.cm.Blues(i) for i in range(len(values))]

💡 Hint

Use a colormap with values between 0 and 1 for gradient.

Option C uses np.linspace to generate evenly spaced values between 0.3 and 1, then maps them to the Blues colormap, producing a gradient.

Practice

1. What does the color parameter do in the plt.bar() function in matplotlib?

easy

A. It changes the color of the bars in the bar chart.

B. It changes the size of the bars.

C. It adds labels to the bars.

D. It changes the style of the axes.

Solution

1. Step 1: Understand the role of plt.bar()

   The plt.bar() function draws bars for a bar chart.

2. Step 2: Identify the color parameter effect

   The color parameter sets the color of the bars, either one color for all or a list for each bar.

3. Final Answer:

   It changes the color of the bars in the bar chart. -> Option A

4. Quick Check:

   color changes bar colors [OK]

Hint: Remember: color = bar colors in plt.bar() [OK]

Common Mistakes:

• Confusing color with size or labels
• Thinking color changes axes or grid
• Using color parameter outside plt.bar()

2. Which of the following is the correct syntax to set all bars to blue in a bar chart using matplotlib?

easy

A. plt.bar(x, height, bar_color='blue')

B. plt.bar(x, height, color='blue')

C. plt.bar(x, height, col='blue')

D. plt.bar(x, height, colors='blue')

Solution

1. Step 1: Recall the correct parameter name

   The parameter to set bar colors is color, not colors, col, or bar_color.

2. Step 2: Check the syntax for setting a single color

   Using color='blue' applies blue color to all bars.

3. Final Answer:

   plt.bar(x, height, color='blue') -> Option B

4. Quick Check:

   Use color parameter exactly [OK]

Hint: Use exact parameter name: color='colorname' [OK]

Common Mistakes:

• Using 'colors' instead of 'color'
• Using 'col' or 'bar_color' which are invalid
• Misspelling 'color' parameter

3. What will be the color of each bar in the following code?

import matplotlib.pyplot as plt
x = [1, 2, 3]
height = [4, 5, 6]
colors = ['red', 'green', 'blue']
plt.bar(x, height, color=colors)
plt.show()

medium

A. Bars will be red, green, and blue respectively.

B. Bars will be blue, green, and red respectively.

C. All bars will be red.

D. The code will cause an error.

Solution

1. Step 1: Understand color list usage in plt.bar()

   When a list of colors is passed to color, each bar gets the corresponding color in order.

2. Step 2: Match colors to bars

   Bars at x=1,2,3 get colors 'red', 'green', 'blue' respectively.

3. Final Answer:

   Bars will be red, green, and blue respectively. -> Option A

4. Quick Check:

   List of colors assigns colors in order [OK]

Hint: List colors in order to color bars individually [OK]

Common Mistakes:

• Assuming all bars get first color only
• Mixing order of colors
• Expecting error from list input

4. Identify the error in this code snippet that tries to color bars individually:

import matplotlib.pyplot as plt
x = [1, 2, 3]
height = [4, 5, 6]
colors = ['red', 'green']
plt.bar(x, height, color=colors)
plt.show()

medium

A. The colors list has fewer colors than bars, causing an error.

B. The colors list should be a tuple, not a list.

C. There is no error; the code runs fine.

D. The color parameter should be named colors.

Solution

1. Step 1: Check length of colors list vs bars

   There are 3 bars but only 2 colors provided.

2. Step 2: Understand matplotlib behavior

   Matplotlib allows a shorter list of colors and cycles through them to color bars.

3. Final Answer:

   There is no error; the code runs fine. -> Option C

4. Quick Check:

   Colors list cycles if shorter than bars [OK]

Hint: Matplotlib cycles colors if list shorter than bars [OK]

Common Mistakes:

• Using fewer colors than bars
• Wrong parameter name 'colors'
• Assuming matplotlib throws error on shorter color list

5. You want to create a bar chart with 5 bars where bars at even positions are blue and odd positions are orange. Which code correctly customizes the bar colors?

hard

A. colors = ['blue', 'orange', 'blue', 'orange', 'blue']
plt.bar(x, height, color=colors)

B. colors = ['blue', 'blue', 'orange', 'orange', 'blue']
plt.bar(x, height, color=colors)

C. colors = ['orange', 'orange', 'blue', 'blue', 'orange']
plt.bar(x, height, color=colors)

D. colors = ['orange', 'blue', 'orange', 'blue', 'orange']
plt.bar(x, height, color=colors)

Solution

1. Step 1: Identify bar positions and colors

   Positions: 1 (odd), 2 (even), 3 (odd), 4 (even), 5 (odd). Odd bars = orange, even bars = blue.

2. Step 2: Map colors to positions in list

   List must be ['orange', 'blue', 'orange', 'blue', 'orange'] to match positions.

3. Final Answer:

   colors = ['orange', 'blue', 'orange', 'blue', 'orange'] plt.bar(x, height, color=colors) -> Option D

4. Quick Check:

   Color list matches odd/even bar positions [OK]

Hint: Assign colors by position index for pattern [OK]

Common Mistakes:

• Swapping colors for odd/even positions
• Using wrong order in colors list
• Not matching list length to bars