Dual y-axis for different scales in Matplotlib - Mini Project: Build & Apply

Dual y-axis for different scales

📖 Scenario: You are analyzing sales and profit data for a small store. Sales are in thousands of dollars, and profit is in hundreds of dollars. You want to show both on the same chart but with different y-axis scales.

🎯 Goal: Create a line chart with two y-axes: one for sales and one for profit, so both can be seen clearly despite different scales.

📋 What You'll Learn

Create lists for sales and profit data with exact values

Create a list for months from January to June

Create a plot with two y-axes using matplotlib

Plot sales on the left y-axis and profit on the right y-axis

Label both y-axes and the x-axis

Add a legend to distinguish sales and profit lines

💡 Why This Matters

🌍 Real World

Dual y-axis charts are useful when comparing two related data sets with different units or scales, like sales and profit, temperature and humidity, or stock price and volume.

💼 Career

Data analysts and scientists often need to visualize multiple metrics together to find insights. Knowing how to create dual y-axis plots helps communicate complex data clearly.

1

Create the data lists

Create a list called months with these exact values: 'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun'. Create a list called sales with these exact values: 20, 35, 30, 35, 27, 40. Create a list called profit with these exact values: 4, 7, 6, 8, 5, 9.

Matplotlib

# Create the lists months, sales, and profit with the exact values
# Your code here

Need a hint?

Use square brackets to create lists. Separate items with commas.

2

Set up the plot and first y-axis

Import matplotlib.pyplot as plt. Create a figure and axis using fig, ax1 = plt.subplots(). Plot sales against months on ax1 with a blue line and label it 'Sales'. Set the x-axis label to 'Month' and the y-axis label of ax1 to 'Sales (thousands)'.

Matplotlib

months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun']
sales = [20, 35, 30, 35, 27, 40]
profit = [4, 7, 6, 8, 5, 9]

# Import matplotlib.pyplot as plt
# Create fig and ax1 with plt.subplots()
# Plot sales on ax1 with blue line and label 'Sales'
# Set x-axis label to 'Month'
# Set y-axis label of ax1 to 'Sales (thousands)'
# Your code here

Need a hint?

Use plt.subplots() to create the plot. Use ax1.plot() to plot sales.

3

Add the second y-axis and plot profit

Create a second y-axis ax2 using ax1.twinx(). Plot profit against months on ax2 with a red line and label it 'Profit'. Set the y-axis label of ax2 to 'Profit (hundreds)' with red color. Add legends for both ax1 and ax2.

Matplotlib

import matplotlib.pyplot as plt

months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun']
sales = [20, 35, 30, 35, 27, 40]
profit = [4, 7, 6, 8, 5, 9]

fig, ax1 = plt.subplots()
ax1.plot(months, sales, 'b-', label='Sales')
ax1.set_xlabel('Month')
ax1.set_ylabel('Sales (thousands)', color='b')

# Create ax2 as a twin of ax1
# Plot profit on ax2 with red line and label 'Profit'
# Set y-axis label of ax2 to 'Profit (hundreds)' with red color
# Add legends for ax1 and ax2
# Your code here

Need a hint?

Use ax1.twinx() to create the second y-axis. Use different colors for clarity.

4

Display the plot

Use plt.show() to display the plot with dual y-axes.

Matplotlib

import matplotlib.pyplot as plt

months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun']
sales = [20, 35, 30, 35, 27, 40]
profit = [4, 7, 6, 8, 5, 9]

fig, ax1 = plt.subplots()
ax1.plot(months, sales, 'b-', label='Sales')
ax1.set_xlabel('Month')
ax1.set_ylabel('Sales (thousands)', color='b')

ax2 = ax1.twinx()
ax2.plot(months, profit, 'r-', label='Profit')
ax2.set_ylabel('Profit (hundreds)', color='r')

ax1.legend(loc='upper left')
ax2.legend(loc='upper right')

# Display the plot
# Your code here

Need a hint?

Use plt.show() to see the plot window.