Dual y-axis for different scales in Matplotlib

Introduction

Sometimes you want to show two sets of data on the same chart but they have very different sizes. Dual y-axis helps by giving each data set its own scale on the left and right sides.

Comparing temperature and rainfall over months on the same plot.

Showing sales numbers and profit margin together.

Plotting stock price and trading volume in one chart.

Visualizing heart rate and speed during exercise.

Syntax

Matplotlib

import matplotlib.pyplot as plt

fig, ax1 = plt.subplots()
ax2 = ax1.twinx()

ax1.plot(x, y1, 'color1')
ax2.plot(x, y2, 'color2')

ax1.set_ylabel('Label for y1')
ax2.set_ylabel('Label for y2')

plt.show()

Use ax1.twinx() to create a second y-axis sharing the same x-axis.

Each axis can have its own labels and colors to keep data clear.

Examples

This example shows two lines with different scales and colors on the same plot.

Matplotlib

import matplotlib.pyplot as plt

x = [1, 2, 3, 4]
y1 = [10, 20, 30, 40]
y2 = [100, 200, 300, 400]

fig, ax1 = plt.subplots()
ax2 = ax1.twinx()

ax1.plot(x, y1, 'g-')
ax2.plot(x, y2, 'b--')

ax1.set_ylabel('Small scale', color='g')
ax2.set_ylabel('Large scale', color='b')

plt.show()

Here, one axis shows squares of numbers, the other shows decreasing values.

Matplotlib

import matplotlib.pyplot as plt

x = range(5)
y1 = [1, 4, 9, 16, 25]
y2 = [100, 80, 60, 40, 20]

fig, ax1 = plt.subplots()
ax2 = ax1.twinx()

ax1.plot(x, y1, 'r-o')
ax2.plot(x, y2, 'b-s')

ax1.set_ylabel('Squares', color='r')
ax2.set_ylabel('Decreasing values', color='b')

plt.show()

Sample Program

This program plots temperature as a red line and rainfall as blue bars on the same chart. Each has its own y-axis scale.

Matplotlib

import matplotlib.pyplot as plt

# Data for plotting
months = ['Jan', 'Feb', 'Mar', 'Apr', 'May']
temperature = [30, 32, 35, 33, 31]  # degrees Celsius
rainfall = [50, 40, 60, 70, 55]     # millimeters

fig, ax1 = plt.subplots()

# Plot temperature on left y-axis
ax1.plot(months, temperature, 'r-o')
ax1.set_ylabel('Temperature (°C)', color='r')
ax1.set_xlabel('Month')

# Create second y-axis for rainfall
ax2 = ax1.twinx()
ax2.bar(months, rainfall, alpha=0.3, color='b')
ax2.set_ylabel('Rainfall (mm)', color='b')

plt.title('Monthly Temperature and Rainfall')
plt.show()

Important Notes

Make sure to use different colors or styles for each axis to avoid confusion.

Dual y-axis can be confusing if overused; keep charts simple and clear.

Summary

Dual y-axis lets you show two data sets with different scales on one plot.

Use ax.twinx() to create the second y-axis.

Label each axis clearly and use different colors for clarity.