Secondary axes in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of plot with secondary y-axis

What will be the output of this code snippet that uses a secondary y-axis in matplotlib?

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3, 4]
y1 = [10, 20, 25, 30]
y2 = [100, 200, 250, 300]
fig, ax1 = plt.subplots()
ax1.plot(x, y1, 'g-')
ax2 = ax1.twinx()
ax2.plot(x, y2, 'b--')
plt.show()

A A plot with one x-axis, left y-axis in green line for y1, and right y-axis in blue dashed line for y2

B A plot with two x-axes and one y-axis showing both y1 and y2 overlapped

C A plot with one x-axis and one y-axis showing only y1 in green line

D A plot with one x-axis and one y-axis showing only y2 in blue dashed line

💡 Hint

Remember that twinx() creates a second y-axis sharing the same x-axis.

The twinx() method creates a secondary y-axis on the right side. The first plot uses the left y-axis (green line), and the second plot uses the right y-axis (blue dashed line).

❓ data_output

intermediate

Number of ticks on secondary y-axis

Given this code, how many major ticks will the secondary y-axis have by default?

Matplotlib

import matplotlib.pyplot as plt
x = [0, 1, 2, 3]
y1 = [1, 4, 9, 16]
y2 = [10, 40, 90, 160]
fig, ax1 = plt.subplots()
ax1.plot(x, y1)
ax2 = ax1.twinx()
ax2.plot(x, y2)
print(len(ax2.get_yticks()))

A 5

B 10

C 7

D 4

💡 Hint

Check the default number of yticks matplotlib creates for a linear scale.

Matplotlib by default creates 7 major ticks on a linear axis unless customized.

❓ visualization

advanced

Identify the correct secondary x-axis transformation

Which option correctly creates a secondary x-axis that shows the square root of the primary x-axis values?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np
x = np.linspace(0, 16, 100)
y = np.sqrt(x)
fig, ax = plt.subplots()
ax.plot(x, y)
secax = ax.secondary_xaxis(1.0, functions=(np.sqrt, np.square))
secax.set_xlabel('Square root of x')
plt.show()

A secax = ax.secondary_xaxis(1.0, functions=(np.sqrt, np.square))

B secax = ax.secondary_xaxis(1.0, functions=(np.square, np.sqrt))

C secax = ax.secondary_xaxis(0.0, functions=(np.sqrt, np.square))

D secax = ax.secondary_xaxis(1.0, functions=(np.log, np.exp))

💡 Hint

The first function converts primary to secondary axis, the second converts back.

The secondary_xaxis method takes two functions: forward and inverse. To show square root on secondary axis, forward is sqrt, inverse is square.

🔧 Debug

advanced

Error when adding secondary y-axis with incompatible scales

What error will this code raise when trying to plot on a secondary y-axis with incompatible data types?

Matplotlib

import matplotlib.pyplot as plt
x = [1, 2, 3]
y1 = [10, 20, 30]
y2 = ['a', 'b', 'c']
fig, ax1 = plt.subplots()
ax1.plot(x, y1)
ax2 = ax1.twinx()
ax2.plot(x, y2)
plt.show()

A ValueError: x and y must have same first dimension

B No error, plot displays with categorical y2

C SyntaxError: invalid syntax

D TypeError: float() argument must be a string or a number, not 'str'

💡 Hint

Check if matplotlib can plot strings as y-values.

Matplotlib cannot convert string values to floats for plotting numeric axes, so it raises a TypeError.

🚀 Application

expert

Using secondary y-axis to compare different units

You have temperature data in Celsius and Fahrenheit for the same dates. Which code snippet correctly plots Celsius on the left y-axis and Fahrenheit on the right y-axis using secondary y-axis?

Matplotlib

import matplotlib.pyplot as plt
dates = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri']
celsius = [0, 10, 20, 15, 5]
fahrenheit = [32, 50, 68, 59, 41]
fig, ax1 = plt.subplots()
ax1.plot(dates, celsius, 'r-', label='Celsius')
ax2 = ax1.twinx()
ax2.plot(dates, fahrenheit, 'b--', label='Fahrenheit')
ax1.set_ylabel('Celsius')
ax2.set_ylabel('Fahrenheit')
plt.show()

A The code runs but both lines overlap on the same y-axis scale

B The code runs correctly and shows Celsius on left y-axis and Fahrenheit on right y-axis

C The code raises TypeError because twinx() cannot be used with string x-axis

D The code raises ValueError because dates are strings and cannot be plotted

💡 Hint

Matplotlib can plot categorical x-axis labels as strings.

Matplotlib supports string x-axis labels and twinx() creates a secondary y-axis. The code correctly plots Celsius and Fahrenheit on separate y-axes.