Date locators for tick spacing in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of matplotlib date locator setting

What will be the number of major ticks on the x-axis after running this code snippet?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import pandas as pd

dates = pd.date_range('2023-01-01', periods=10, freq='D')
values = range(10)

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_locator(mdates.DayLocator(interval=3))
plt.draw()

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

A 4

B 5

C 3

D 10

💡 Hint

Think about how many days are covered and the interval of 3 days for ticks.

The date range covers 10 days from Jan 1 to Jan 10. Using DayLocator with interval=3 places ticks every 3 days starting from the first date: Jan 1, 4, 7, 10. This results in 4 ticks.

❓ data_output

intermediate

Resulting tick labels with MonthLocator

Given this code, what will be the list of tick labels shown on the x-axis?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import pandas as pd

dates = pd.date_range('2023-01-01', periods=90, freq='D')
values = range(90)

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_locator(mdates.MonthLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%b'))
plt.draw()

labels = [label.get_text() for label in ax.get_xticklabels() if label.get_text()]
print(labels)

A ['Jan', 'Feb']

B ['Jan', 'Feb', 'Mar']

C ['Jan']

D ['Feb', 'Mar']

💡 Hint

Check the date range and which months are included.

The date range covers 90 days starting Jan 1, 2023, up to Mar 31, including January, February, and March. MonthLocator places ticks at the start of each month: Jan 1, Feb 1, Mar 1. The formatter shows '%b': ['Jan', 'Feb', 'Mar'].

❓ visualization

advanced

Visual effect of using WeekdayLocator with different intervals

Which option shows the correct number of major ticks on the x-axis when using WeekdayLocator with interval=2 on a 30-day date range?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import pandas as pd

dates = pd.date_range('2023-01-01', periods=30, freq='D')
values = range(30)

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_locator(mdates.WeekdayLocator(byweekday=mdates.MO, interval=2))
plt.draw()

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

A 3

B 4

C 5

D 6

💡 Hint

Count Mondays every 2 weeks within 30 days.

Jan 1, 2023 is Sunday. Mondays: Jan 2, 9, 16, 23, 30. WeekdayLocator(byweekday=MO, interval=2) places ticks every 2 weeks on Mondays starting from the first: Jan 2, 16, 30. Total 3 ticks.

🔧 Debug

advanced

Identify the error in this date locator usage

What error will this code raise when setting the major locator?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import pandas as pd

dates = pd.date_range('2023-01-01', periods=5, freq='D')
values = range(5)

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_locator(mdates.MonthLocator(interval='2'))
plt.show()

A AttributeError: 'str' object has no attribute 'days'

B No error, runs successfully

C ValueError: interval must be an integer

D TypeError: unsupported operand type(s) for +: 'int' and 'str'

💡 Hint

Check the type of the interval argument.

MonthLocator expects an integer for interval. Passing '2' (string) causes a TypeError during tick computation when adding it to an integer (e.g., month calculation).

🚀 Application

expert

Choosing the correct locator for quarterly data visualization

You have a time series dataset with daily data spanning multiple years. You want to place major ticks at the start of each quarter and minor ticks at the start of each month. Which combination of locators should you use?

A Major: mdates.MonthLocator() and Minor: mdates.MonthLocator(bymonth=[1,4,7,10])

B Major: mdates.MonthLocator(bymonth=[1,4,7,10]) and Minor: mdates.MonthLocator()

C Major: mdates.MonthLocator(bymonth=[1,4,7,10]) and Minor: mdates.MonthLocator(bymonth=[2,3,5,6,8,9,11,12])

D Major: mdates.MonthLocator(bymonth=[1,4,7,10]) and Minor: mdates.DayLocator(interval=30)

💡 Hint

Quarters start in months 1,4,7,10. Minor ticks should be other months.

To place major ticks at the start of each quarter, use MonthLocator with bymonth=[1,4,7,10]. For minor ticks at the start of other months, use MonthLocator with bymonth set to the remaining months. This ensures clear separation of major and minor ticks.