Date formatting with mdates in Matplotlib - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of mdates in matplotlib?

mdates helps format and handle dates on plots, making it easier to display time-based data clearly.

beginner

How do you set a date format for the x-axis using mdates?

You use mdates.DateFormatter with a format string like '%Y-%m-%d' and apply it with ax.xaxis.set_major_formatter().

intermediate

What does mdates.AutoDateLocator() do?

It automatically chooses the best date ticks for the axis based on the data range, so the dates look neat and readable.

beginner

Why is it important to use mdates for date plotting instead of plain numbers?

Dates are special data types. Using mdates ensures correct spacing, formatting, and interpretation of dates on plots.

beginner

Show a simple code snippet to format dates on the x-axis with mdates.

<pre>import matplotlib.pyplot as plt
import matplotlib.dates as mdates
from datetime import datetime

dates = [datetime(2024, 1, i) for i in range(1, 6)]
values = [10, 15, 7, 12, 9]

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_locator(mdates.DayLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))
fig.autofmt_xdate()  # Rotate date labels
plt.show()</pre>

Which mdates class automatically picks date ticks based on data range?

A DateFormatter

B AutoDateLocator

C DayLocator

D DateConverter

AutoDateLocator automatically selects the best ticks for dates on the axis.

What does DateFormatter('%Y-%m-%d') do?

A Formats dates as Year-Month-Day

B Locates dates on the axis

C Converts strings to dates

D Rotates date labels

It formats the date labels to show year, month, and day.

Which method applies the date format to the x-axis in matplotlib?

A ax.xaxis.set_major_formatter()

B ax.set_xlabel()

C plt.date_format()

D ax.xaxis.set_major_locator()

set_major_formatter() sets how the major ticks are displayed, including date formats.

Why use fig.autofmt_xdate() after setting date formats?

A To add grid lines

B To change the color of dates

C To rotate date labels for better readability

D To zoom into the date range

This rotates the date labels so they don’t overlap and are easier to read.

Which mdates locator would you use to show ticks every day?

A YearLocator

B MonthLocator

C AutoDateLocator

D DayLocator

DayLocator places ticks at daily intervals.

Explain how to format dates on the x-axis of a matplotlib plot using mdates.

Think about importing mdates, setting locator and formatter, and rotating labels.

Why is it better to use mdates for date plotting instead of plotting dates as strings or numbers?

Consider how dates behave differently from plain numbers on a plot.

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