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.

Practice

1. What is the main purpose of using DateLocator classes in matplotlib when plotting dates?

easy

A. To add grid lines to the plot

B. To change the color of the plot lines

C. To set the plot title automatically

D. To control where date ticks appear on the axis

Solution

1. Step 1: Understand the role of DateLocator

   DateLocator classes in matplotlib are designed to specify the positions of ticks on date axes.

2. Step 2: Identify the correct purpose

   They do not affect colors, grid lines, or titles but control tick placement for dates.

3. Final Answer:

   To control where date ticks appear on the axis -> Option D

4. Quick Check:

   DateLocator controls tick positions = C [OK]

Hint: DateLocator sets tick positions on date axes [OK]

Common Mistakes:

• Confusing tick placement with plot styling
• Thinking DateLocator changes colors or titles
• Assuming DateLocator adds grid lines

2. Which of the following is the correct way to import the MonthLocator from matplotlib's dates module?

easy

A. import matplotlib.MonthLocator

B. from matplotlib.dates import MonthLocator

C. from matplotlib import MonthLocator

D. import MonthLocator from matplotlib.dates

Solution

1. Step 1: Recall correct import syntax for specific classes

   In Python, to import a class from a submodule, use 'from module import ClassName'.

2. Step 2: Match with matplotlib.dates module

   MonthLocator is in matplotlib.dates, so the correct import is 'from matplotlib.dates import MonthLocator'.

3. Final Answer:

   from matplotlib.dates import MonthLocator -> Option B

4. Quick Check:

   Correct import syntax = A [OK]

Hint: Use 'from module import Class' for specific imports [OK]

Common Mistakes:

• Using 'import' without 'from' for submodules
• Incorrect module path or syntax
• Confusing import order or keywords

3. What will be the effect of this code snippet on the x-axis ticks?

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

dates = pd.date_range('2024-01-01', periods=10)
values = range(10)

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

medium

A. Ticks will appear every 3 days on the x-axis

B. Ticks will appear every day on the x-axis

C. Ticks will appear every 3 months on the x-axis

D. Ticks will not appear on the x-axis

Solution

1. Step 1: Understand DayLocator with interval=3

   DayLocator places ticks on days. Setting interval=3 means ticks every 3 days.

2. Step 2: Apply to the plot

   The x-axis will show ticks spaced by 3 days along the date range.

3. Final Answer:

   Ticks will appear every 3 days on the x-axis -> Option A

4. Quick Check:

   DayLocator(interval=3) = ticks every 3 days [OK]

Hint: DayLocator(interval=3) means ticks every 3 days [OK]

Common Mistakes:

• Confusing days with months or weeks
• Ignoring the interval parameter
• Expecting no ticks due to interval

4. Identify the error in this code that tries to set weekly ticks on Mondays:

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

dates = pd.date_range('2024-01-01', periods=14)
values = range(14)

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_locator(mdates.WeekdayLocator(byweekday=mdates.MONDAY, interval=1))
plt.show()

medium

A. Dates must be strings, not pandas Timestamps

B. WeekdayLocator does not accept byweekday argument

C. mdates.MONDAY does not exist; use mdates.MO instead

D. interval cannot be set to 1

Solution

1. Step 1: Check WeekdayLocator constants

   matplotlib.dates does not define MONDAY; the constant for Monday is MO (which is 0).

2. Step 2: Identify correct usage

   Replace mdates.MONDAY with mdates.MO. A single weekday constant does not need a list or tuple.

3. Final Answer:

   mdates.MONDAY does not exist; use mdates.MO instead -> Option C

4. Quick Check:

   mdates.MO for Monday (not MONDAY) = D [OK]

Hint: mdates.MO for Monday, not MONDAY [OK]

Common Mistakes:

• Using MONDAY instead of MO
• Misunderstanding weekday constant names (MO, TU, etc.)
• Confusing with full day names like dateutil

5. You want to plot daily data for 6 months but only show ticks on the first day of each month. Which locator and interval setting achieves this best?

hard

A. Use MonthLocator(bymonthday=1, interval=1)

B. Use DayLocator(interval=30)

C. Use WeekdayLocator(byweekday=mdates.MONDAY, interval=4)

D. Use MonthLocator(bymonthday=15, interval=1)

Solution

1. Step 1: Understand requirement for ticks on first day of each month

   MonthLocator with bymonthday=1 places ticks on the first day of each month.

2. Step 2: Check interval for monthly spacing

   Setting interval=1 ensures ticks appear every month, exactly on day 1.

3. Final Answer:

   Use MonthLocator(bymonthday=1, interval=1) -> Option A

4. Quick Check:

   MonthLocator with bymonthday=1 = ticks on first day monthly [OK]

Hint: MonthLocator with bymonthday=1 shows first day ticks [OK]

Common Mistakes:

• Using DayLocator with fixed interval ignores month boundaries
• Choosing wrong day like 15 instead of 1
• Using WeekdayLocator which targets weekdays, not month start