Colorbar formatting in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this colorbar tick labels code?

Consider this code snippet that creates a colorbar with custom tick labels. What will be the printed list of tick labels?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()
img = ax.imshow(np.random.rand(5,5), cmap='viridis')
cbar = fig.colorbar(img)
cbar.set_ticks([0.2, 0.5, 0.8])
ticks = cbar.ax.get_yticklabels()
labels = [tick.get_text() for tick in ticks]
print(labels)

A ['0', '0.5', '1']

B ['0.20', '0.50', '0.80']

C ['', '', '']

D ['0.2', '0.5', '0.8']

💡 Hint

Remember that get_yticklabels() returns label objects which may not have text set until drawn.

The get_yticklabels() method returns label objects but their text is empty until the figure is drawn. So the printed list is empty strings.

❓ data_output

intermediate

How many ticks will this colorbar display?

This code creates a colorbar with a fixed number of ticks. How many ticks will appear on the colorbar?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()
img = ax.imshow(np.random.rand(10,10), cmap='plasma')
cbar = fig.colorbar(img, ticks=np.linspace(0,1,5))
print(len(cbar.ax.get_yticks()))

A 4

B 6

C 10

D 5

💡 Hint

Check how many values are in np.linspace(0,1,5).

The ticks argument sets exactly 5 ticks, so get_yticks() returns 5 values.

🔧 Debug

advanced

Why does this colorbar label formatting code raise an error?

Look at this code that tries to format colorbar tick labels. What error does it raise?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()
img = ax.imshow(np.random.rand(5,5), cmap='cool')
cbar = fig.colorbar(img)
cbar.ax.yaxis.set_major_formatter('{:.2f}')

A TypeError: 'str' object is not callable

B AttributeError: 'str' object has no attribute 'format'

C ValueError: Invalid format string

D No error, runs fine

💡 Hint

Check what type set_major_formatter expects as argument.

The set_major_formatter method expects a Formatter object, not a string. Passing a string causes a TypeError because strings are not callable.

🚀 Application

advanced

Which code correctly formats colorbar ticks as percentages?

You want to show colorbar tick labels as percentages (e.g., 20%, 50%). Which option correctly formats the colorbar ticks?

Matplotlib

import matplotlib.pyplot as plt
import numpy as np
from matplotlib.ticker import FuncFormatter

fig, ax = plt.subplots()
img = ax.imshow(np.random.rand(5,5), cmap='inferno')
cbar = fig.colorbar(img)

A cbar.ax.yaxis.set_major_formatter(FuncFormatter(lambda x, _: f'{int(x*100)}%'))

B

cbar.formatter = FuncFormatter(lambda x, _: f'{x*100}%')
cbar.update_ticks()

C cbar.ax.yaxis.set_major_formatter('{:.0%}')

D

cbar.set_ticks([0, 0.5, 1])
cbar.ax.set_yticklabels(['0%', '50%', '100%'])

💡 Hint

Use FuncFormatter to create a custom label format function.

Option A uses FuncFormatter with a lambda that multiplies the tick value by 100 and adds a percent sign, which is the correct way.

Option A manually sets labels but does not update ticks properly.

Option A passes a string instead of a Formatter object, causing error.

Option A tries to assign formatter attribute directly, which is not correct usage.

🧠 Conceptual

expert

What is the effect of calling cbar.ax.invert_yaxis() on a vertical colorbar?

In matplotlib, what happens when you call invert_yaxis() on the colorbar's axis?

A The colorbar disappears from the plot.

B The colorbar colors are reversed vertically, so high values appear at the bottom.

C The colorbar tick labels are rotated 180 degrees but colors stay the same.

D The colorbar tick labels become bold but colors stay the same.

💡 Hint

Think about what inverting the y-axis means visually.

Calling invert_yaxis() flips the vertical axis, so the color gradient is reversed vertically. High values appear at the bottom instead of top.