Date formatting with mdates in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the x-axis date format shown by this code?

This code plots dates on the x-axis using matplotlib and mdates. What date format will appear on the x-axis labels?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import datetime

dates = [datetime.datetime(2024, 1, i+1) for i in range(5)]
values = [10, 20, 15, 25, 30]

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))
plt.show()

A Dates formatted as 'DD-MM-YYYY', e.g., '01-01-2024'

B Dates formatted as 'MM/DD/YYYY', e.g., '01/01/2024'

C Dates formatted as 'Month day, Year', e.g., 'January 1, 2024'

D Dates formatted as 'YYYY-MM-DD', e.g., '2024-01-01'

💡 Hint

Look at the string inside DateFormatter to see the date format pattern.

The DateFormatter is set with '%Y-%m-%d', which means year-month-day with dashes, so the dates appear like '2024-01-01'.

❓ data_output

intermediate

How many major ticks appear on the x-axis?

Given this code that plots 10 daily dates and sets the major locator to mdates.DayLocator(interval=3), how many major ticks will appear on the x-axis?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import datetime

dates = [datetime.datetime(2024, 3, i+1) for i in range(10)]
values = range(10)

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_locator(mdates.DayLocator(interval=3))
plt.close(fig)  # Close plot to avoid display

A 4

B 3

C 5

D 10

💡 Hint

Count days from 1 to 10 with steps of 3: days 1,4,7,10.

DayLocator with interval=3 places ticks every 3 days starting from the first date, so ticks appear on days 1,4,7,10, totaling 4 ticks.

🔧 Debug

advanced

Why does this code raise a TypeError?

This code tries to format dates on the x-axis but raises a TypeError. What is the cause?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import datetime

dates = ['2024-01-01', '2024-01-02', '2024-01-03']
values = [1, 2, 3]

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))
plt.show()

A Dates are strings, but DateFormatter expects datetime objects, causing TypeError.

B The DateFormatter format string '%Y-%m-%d' is invalid and causes TypeError.

C The plot function cannot accept lists as x-axis values, causing TypeError.

D Missing plt.show() causes TypeError when rendering the plot.

💡 Hint

Check the type of the dates list elements.

DateFormatter expects datetime objects to format dates. Passing strings causes a TypeError because it cannot format string objects as dates.

❓ visualization

advanced

Which option correctly sets monthly ticks with abbreviated month names?

You want the x-axis to show ticks at the start of each month with labels like 'Jan', 'Feb', etc. Which code snippet achieves this?

A

ax.xaxis.set_major_locator(mdates.WeekdayLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%b'))

B

ax.xaxis.set_major_locator(mdates.MonthLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%b'))

C

ax.xaxis.set_major_locator(mdates.MonthLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%B'))

D

ax.xaxis.set_major_locator(mdates.DayLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%b'))

💡 Hint

Look for MonthLocator and '%b' for abbreviated month names.

MonthLocator sets ticks at month starts. DateFormatter with '%b' formats months as abbreviated names like 'Jan'.

🧠 Conceptual

expert

What happens if you set both major and minor locators with mdates but only format major ticks?

If you set ax.xaxis.set_major_locator(mdates.MonthLocator()) and ax.xaxis.set_minor_locator(mdates.WeekdayLocator()), but only set a major formatter with DateFormatter, what will the minor ticks show?

A Minor ticks cause an error because they lack a formatter.

B Minor ticks show the same labels as major ticks, causing label overlap.

C Minor ticks appear without labels because no formatter is set for them.

D Minor ticks are hidden automatically if no formatter is set.

💡 Hint

Think about how matplotlib handles tick labels when no formatter is assigned.

Minor ticks appear as marks but have no labels if no formatter is assigned. Only major ticks get labels from the major formatter.

Practice

1. What is the main purpose of using mdates.DateFormatter in matplotlib?

easy

A. To customize how dates appear on the x-axis of a plot

B. To change the color of the plot lines

C. To add grid lines to the plot

D. To save the plot as an image file

Solution

1. Step 1: Understand the role of DateFormatter

   mdates.DateFormatter is used to format date labels on axes in matplotlib plots.

2. Step 2: Identify what DateFormatter customizes

   It changes how dates are shown, for example, showing only month and day or full date and time.

3. Final Answer:

   To customize how dates appear on the x-axis of a plot -> Option A

4. Quick Check:

   DateFormatter formats dates = A [OK]

Hint: DateFormatter changes date label style on axes [OK]

Common Mistakes:

• Confusing DateFormatter with plot styling functions
• Thinking it changes plot colors
• Assuming it saves files

2. Which of the following is the correct way to create a date formatter that shows dates as 'Year-Month-Day' using mdates.DateFormatter?

easy

A. mdates.DateFormatter('Y-m-d')

B. mdates.DateFormatter('%D-%M-%Y')

C. mdates.DateFormatter('%y/%M/%d')

D. mdates.DateFormatter('%Y-%m-%d')

Solution

1. Step 1: Recall strftime format codes

   Year is '%Y', month is '%m', day is '%d' in strftime formatting.

2. Step 2: Match correct format string

   mdates.DateFormatter('%Y-%m-%d') uses '%Y-%m-%d' which correctly formats as Year-Month-Day.

3. Final Answer:

   mdates.DateFormatter('%Y-%m-%d') -> Option D

4. Quick Check:

   Correct strftime codes = B [OK]

Hint: Use %Y, %m, %d for year, month, day in DateFormatter [OK]

Common Mistakes:

• Omitting % signs in format string
• Using uppercase M for minutes instead of month
• Mixing order of year, month, day

3. What will be the output on the x-axis labels if the following code is used?

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

dates = pd.date_range('2023-01-01', periods=3)
values = [10, 20, 15]

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_formatter(mdates.DateFormatter('%b %d'))
plt.show()

medium

A. Labels show dates as '01-01-2023', '01-02-2023', '01-03-2023'

B. Labels show dates as 'Jan 01', 'Jan 02', 'Jan 03'

C. Labels show dates as '2023-01-01', '2023-01-02', '2023-01-03'

D. Labels show dates as '01 Jan', '02 Jan', '03 Jan'

Solution

1. Step 1: Understand the format string '%b %d'

   '%b' is abbreviated month name (e.g., Jan), '%d' is day with leading zero.

2. Step 2: Apply format to dates

   Dates will appear as 'Jan 01', 'Jan 02', 'Jan 03' on x-axis labels.

3. Final Answer:

   Labels show dates as 'Jan 01', 'Jan 02', 'Jan 03' -> Option B

4. Quick Check:

   Format '%b %d' = 'Jan 01' style [OK]

Hint: Check strftime codes to predict label format [OK]

Common Mistakes:

• Confusing %b with %B (full month name)
• Expecting numeric month instead of abbreviated
• Ignoring leading zeros in day

4. Identify the error in this code snippet that tries to format dates on the x-axis:

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

dates = pd.date_range('2023-01-01', periods=3)
values = [5, 10, 15]

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))
ax.xaxis.set_major_locator(mdates.DayLocator())
plt.show()

medium

A. No error; code runs and formats dates correctly

B. Missing import for DayLocator

C. DateFormatter format string is invalid

D. Plot will fail because dates are not converted to numbers

Solution

1. Step 1: Check imports and usage

   All necessary imports are present, including mdates.DayLocator and DateFormatter.

2. Step 2: Verify date formatting and locator

   DateFormatter uses valid '%Y-%m-%d' format; DayLocator sets ticks daily. Dates are pandas datetime objects, which matplotlib handles.

3. Final Answer:

   No error; code runs and formats dates correctly -> Option A

4. Quick Check:

   Correct imports and usage = D [OK]

Hint: Check imports and valid format strings first [OK]

Common Mistakes:

• Assuming pandas dates need manual conversion
• Using wrong format codes in DateFormatter
• Forgetting to set locator for ticks

5. You want to plot daily sales data for a month and show x-axis labels only for Mondays in 'Month-Day' format (e.g., 'Jan-02'). Which combination of mdates functions should you use to achieve this?

hard

A. Use mdates.MonthLocator() and mdates.DateFormatter('%b')

B. Use mdates.DayLocator() and mdates.DateFormatter('%Y-%m-%d')

C. Use mdates.WeekdayLocator(byweekday=mdates.MO) and mdates.DateFormatter('%b-%d')

D. Use mdates.HourLocator() and mdates.DateFormatter('%H:%M')

Solution

1. Step 1: Choose locator for Mondays only

   mdates.WeekdayLocator(byweekday=mdates.MO) selects ticks only on Mondays.

2. Step 2: Format labels as 'Month-Day'

   mdates.DateFormatter('%b-%d') formats dates like 'Jan-02'.

3. Final Answer:

   Use mdates.WeekdayLocator(byweekday=mdates.MO) and mdates.DateFormatter('%b-%d') -> Option C

4. Quick Check:

   Monday ticks + 'Jan-02' format = C [OK]

Hint: Use WeekdayLocator with MO and DateFormatter '%b-%d' [OK]

Common Mistakes:

• Using DayLocator shows all days, not Mondays only
• MonthLocator shows monthly ticks, not weekly
• HourLocator is for time, not dates