Diverging bar charts in Matplotlib

Introduction

Diverging bar charts help us compare positive and negative values side by side. They make it easy to see which items are above or below a baseline.

Comparing survey results with positive and negative responses.

Showing profit and loss for different products.

Visualizing changes that go up or down from zero.

Highlighting differences in ratings above or below average.

Syntax

Matplotlib

import matplotlib.pyplot as plt

# Data
labels = ['A', 'B', 'C', 'D']
values = [10, -5, 15, -10]

# Plot
plt.bar(labels, values, color=['green' if v >= 0 else 'red' for v in values])
plt.axhline(0, color='black')
plt.show()

Use colors to separate positive and negative bars for clarity.

Draw a horizontal line at zero to show the baseline.

Examples

Simple diverging bar chart with custom colors for positive and negative values.

Matplotlib

import matplotlib.pyplot as plt

labels = ['Jan', 'Feb', 'Mar']
values = [20, -10, 15]

colors = ['blue' if v >= 0 else 'orange' for v in values]
plt.bar(labels, values, color=colors)
plt.axhline(0, color='gray')
plt.show()

Adding a title to the diverging bar chart for better understanding.

Matplotlib

import matplotlib.pyplot as plt

labels = ['X', 'Y', 'Z']
values = [-5, 10, -3]

plt.bar(labels, values, color=['red' if v < 0 else 'green' for v in values])
plt.axhline(0, color='black')
plt.title('Diverging Bar Chart')
plt.show()

Sample Program

This program creates a diverging bar chart showing positive and negative values for products. Green bars go up, red bars go down, and a black line marks zero.

Matplotlib

import matplotlib.pyplot as plt

# Categories and their values
categories = ['Product A', 'Product B', 'Product C', 'Product D']
values = [30, -20, 15, -10]

# Choose colors: green for positive, red for negative
colors = ['green' if v >= 0 else 'red' for v in values]

# Create bar chart
plt.bar(categories, values, color=colors)

# Draw baseline at zero
plt.axhline(0, color='black')

# Add labels and title
plt.ylabel('Value')
plt.title('Diverging Bar Chart Example')

# Show plot
plt.show()

Important Notes

Always include a zero baseline line to separate positive and negative bars clearly.

Use contrasting colors for positive and negative bars to make the chart easy to read.

Keep labels short and clear to avoid clutter.

Summary

Diverging bar charts show positive and negative values side by side.

Use colors and a zero line to make differences clear.

They are great for comparing gains and losses or opinions above and below a midpoint.

Practice

1. What is the main purpose of a diverging bar chart in data visualization?

easy

A. To visualize hierarchical data using nested bars

B. To show positive and negative values side by side for easy comparison

C. To plot time series data with a line connecting points

D. To display only positive values in a stacked format

Solution

1. Step 1: Understand the chart type

   Diverging bar charts are designed to display data that has both positive and negative values.

2. Step 2: Identify the purpose

   They place positive and negative bars side by side to highlight differences clearly.

3. Final Answer:

   To show positive and negative values side by side for easy comparison -> Option B

4. Quick Check:

   Diverging bar chart = positive & negative side by side [OK]

Hint: Look for charts showing positive and negative bars side by side [OK]

Common Mistakes:

• Confusing diverging bar charts with stacked bar charts
• Thinking they only show positive values
• Mixing up with line charts

2. Which of the following is the correct way to create a diverging bar chart using matplotlib's barh function for negative and positive values?

easy

A. plt.barh(y_pos, abs(values), color='red')

B. plt.barh(y_pos, values, color='blue')

C. plt.barh(y_pos, values, color=['red' if v < 0 else 'green' for v in values])

D. plt.barh(y_pos, values, color=['green' if v > 0 else 'blue' for v in values])

Solution

1. Step 1: Understand color assignment for diverging bars

   Colors should differ based on whether values are positive or negative to highlight divergence.

2. Step 2: Check the color list comprehension

   plt.barh(y_pos, values, color=['red' if v < 0 else 'green' for v in values]) uses a list comprehension assigning 'red' for negative and 'green' for positive values, which is correct.

3. Final Answer:

   plt.barh(y_pos, values, color=['red' if v < 0 else 'green' for v in values]) -> Option C

4. Quick Check:

   Color by sign = plt.barh(y_pos, values, color=['red' if v < 0 else 'green' for v in values]) [OK]

Hint: Use list comprehension to color bars by sign [OK]

Common Mistakes:

• Using a single color for all bars
• Taking absolute values which lose sign info
• Assigning wrong colors to positive/negative values

3. Given the code below, what will be the color of the bar corresponding to the value -3?

import matplotlib.pyplot as plt
values = [4, -3, 2, -1]
y_pos = range(len(values))
colors = ['green' if v > 0 else 'red' for v in values]
plt.barh(y_pos, values, color=colors)
plt.show()

medium

A. No color (default)

B. Green

C. Blue

D. Red

Solution

1. Step 1: Analyze the color assignment logic

   The list comprehension assigns 'green' if value > 0, else 'red'.

2. Step 2: Check the value -3

   -3 is less than 0, so it gets 'red'.

3. Final Answer:

   Red -> Option D

4. Quick Check:

   Negative value color = red [OK]

Hint: Negative values get 'red' color here [OK]

Common Mistakes:

• Assuming negative values get green
• Confusing color assignment logic
• Ignoring the else condition in list comprehension

4. The following code is intended to create a diverging bar chart but does not color bars correctly. What is the error?

values = [5, -2, 3, -4]
y_pos = range(len(values))
colors = ['green' if v < 0 else 'red' for v in values]
plt.barh(y_pos, values, color=colors)
plt.show()

medium

A. The color condition is reversed; positive values should be green

B. The y_pos variable should be a list, not a range

C. The barh function cannot accept a color list

D. Values should be absolute for coloring

Solution

1. Step 1: Review the color condition

   The code assigns 'green' if value < 0, else 'red'. This means negative values are green and positive are red.

2. Step 2: Understand diverging bar chart color convention

   Usually, positive values are green and negative values are red to show gains and losses clearly.

3. Final Answer:

   The color condition is reversed; positive values should be green -> Option A

4. Quick Check:

   Positive = green, negative = red [OK]

Hint: Positive bars usually green, negative bars red [OK]

Common Mistakes:

• Reversing color conditions
• Thinking range is invalid for y_pos
• Using absolute values for coloring

5. You have a dataset with sales gains and losses for 5 products: {'A': 10, 'B': -5, 'C': 0, 'D': 7, 'E': -3}. How would you create a diverging bar chart that clearly shows gains in green, losses in red, and zero values in gray using matplotlib?

hard

A. Use a color list with a condition: green if >0, red if <0, gray if 0, then plot with plt.barh

B. Plot absolute values only with a single color to avoid confusion

C. Use plt.barh with colors assigned randomly

D. Plot only positive values and ignore zeros and negatives

Solution

1. Step 1: Define color conditions for all cases

   Assign 'green' for positive, 'red' for negative, and 'gray' for zero values to distinguish all clearly.

2. Step 2: Create color list and plot

   Use a list comprehension to assign colors based on value sign, then plot with plt.barh using these colors.

3. Final Answer:

   Use a color list with a condition: green if >0, red if <0, gray if 0, then plot with plt.barh -> Option A

4. Quick Check:

   Color by sign including zero = Use a color list with a condition: green if >0, red if <0, gray if 0, then plot with plt.barh [OK]

Hint: Add gray for zero values in color list [OK]

Common Mistakes:

• Ignoring zero values color
• Using absolute values losing sign info
• Assigning random colors without logic