Custom tick formatters in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a custom tick formatter using FuncFormatter

What is the output of the following code snippet that uses FuncFormatter to format y-axis ticks as percentages?

Matplotlib

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

fig, ax = plt.subplots()
ax.plot([0, 1, 2], [0.1, 0.5, 0.9])

formatter = FuncFormatter(lambda x, _: f'{x*100:.0f}%')
ax.yaxis.set_major_formatter(formatter)

ticks = [tick.get_text() for tick in ax.yaxis.get_major_ticks()]
print(ticks)

A ['0.1', '0.3', '0.5', '0.7', '0.9']

B ['0%', '20%', '40%', '60%', '80%', '100%']

C ['10%', '50%', '90%']

D ['0', '0.2', '0.4', '0.6', '0.8', '1.0']

💡 Hint

Think about how the formatter multiplies the tick value by 100 and formats it as a percentage with no decimals.

The FuncFormatter multiplies each tick value by 100 and formats it as an integer percentage. The default major ticks on the y-axis are at 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0, so the formatted labels become '0%', '20%', '40%', '60%', '80%', and '100%'.

❓ data_output

intermediate

Number of ticks after setting a custom formatter

After applying a custom tick formatter that formats x-axis ticks as integers with a prefix 'Val-', how many major ticks will be shown on the x-axis for the following plot?

Matplotlib

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

fig, ax = plt.subplots()
ax.plot(range(10), range(10))

formatter = FuncFormatter(lambda x, _: f'Val-{int(x)}')
ax.xaxis.set_major_formatter(formatter)

num_ticks = len(ax.get_xticks())
print(num_ticks)

A 9

B 10

C 12

D 11

💡 Hint

Consider the default ticks matplotlib generates for the range 0 to 9.

Matplotlib by default places 11 ticks for the range 0 to 9 (including 0 and 10). The formatter only changes the label appearance, not the number of ticks. So the number of ticks remains 11.

🔧 Debug

advanced

Identify the error in custom tick formatter code

What error will the following code raise when setting a custom tick formatter?

Matplotlib

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

fig, ax = plt.subplots()
ax.plot([1, 2, 3], [4, 5, 6])

def formatter(x):
    return f'Value: {x:.1f}'

ax.yaxis.set_major_formatter(FuncFormatter(formatter))

A TypeError: formatter() takes 1 positional argument but 2 were given

B No error, runs correctly

C ValueError: unsupported format string passed to float.__format__

D SyntaxError: invalid syntax in formatter function

💡 Hint

Check the expected signature of the function passed to FuncFormatter.

The function passed to FuncFormatter must accept two arguments: the tick value and the tick position. The provided formatter function only accepts one argument, so Python raises a TypeError because it is called with two arguments.

❓ visualization

advanced

Effect of a custom log scale tick formatter

Given the code below, what will the y-axis tick labels look like?

Matplotlib

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

fig, ax = plt.subplots()
ax.plot(np.logspace(0.1, 2, 10))

formatter = FuncFormatter(lambda y, _: f'{y:.1e}')
ax.set_yscale('log')
ax.yaxis.set_major_formatter(formatter)

labels = [tick.get_text() for tick in ax.yaxis.get_major_ticks()]
print(labels)

A ['1.0e+00', '1.0e+01', '1.0e+02']

B ['0.1', '1.0', '10.0', '100.0']

C ['0.1e+00', '1.0e+00', '10.0e+00']

D ['1', '10', '100']

💡 Hint

Remember that the formatter formats the tick values in scientific notation with one decimal place.

The y-axis is set to log scale, so the major ticks are at powers of 10 (1, 10, 100). The formatter formats these values in scientific notation with one decimal place, resulting in labels like '1.0e+00', '1.0e+01', and '1.0e+02'.

🚀 Application

expert

Creating a custom tick formatter for time in seconds to MM:SS format

You want to display x-axis ticks representing time in seconds as MM:SS format (minutes and seconds). Which of the following FuncFormatter functions will correctly format the ticks?

A lambda x: f'{int(x//60)}:{int(x%60)}'

B lambda x, pos: f'{x//60}:{x%60}'

C lambda x, _: f'{int(x//60):02d}:{int(x%60):02d}'

D lambda x, _: f'{round(x/60)}:{round(x%60)}'

💡 Hint

Remember the formatter function must accept two arguments and format minutes and seconds with leading zeros.

Option C correctly defines a function with two arguments (required by FuncFormatter) and formats the time in MM:SS with leading zeros using int() and :02d formatting. Other options either have wrong arguments, missing leading zeros, or incorrect rounding.