Dates on x-axis in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this matplotlib date plot code?

Consider the following Python code that plots dates on the x-axis using matplotlib. What will be the format of the x-axis labels shown in the plot?

Matplotlib

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

dates = [datetime(2024, 1, 1), datetime(2024, 1, 2), datetime(2024, 1, 3)]
values = [10, 20, 15]

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

A Dates formatted as 'YYYY-MM-DD', e.g., '2024-01-01', '2024-01-02', '2024-01-03'

B Dates formatted as 'MM/DD/YYYY', e.g., '01/01/2024', '01/02/2024', '01/03/2024'

C Dates formatted as 'Day Month Year', e.g., '1 Jan 2024', '2 Jan 2024', '3 Jan 2024'

D Dates shown as integers representing days since 1970-01-01

💡 Hint

Look at the format string passed to DateFormatter.

The DateFormatter is set with the format '%Y-%m-%d', which means year-month-day with four-digit year, two-digit month, and two-digit day separated by dashes.

❓ data_output

intermediate

How many major ticks appear on the x-axis?

Given this code that plots data with dates on the x-axis, how many major ticks will appear on the x-axis?

Matplotlib

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

dates = [datetime(2024, 5, 1), datetime(2024, 5, 2), datetime(2024, 5, 3), datetime(2024, 5, 4), datetime(2024, 5, 5)]
values = [5, 3, 6, 2, 7]

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

A 3 major ticks

B 5 major ticks

C 2 major ticks

D 1 major tick

💡 Hint

Check the interval set in DayLocator and the date range.

DayLocator with interval=2 places a major tick every 2 days starting from the first date. For 5 days, ticks appear on day 1, 3, and 5, totaling 3 ticks.

❓ visualization

advanced

Which plot shows correctly formatted monthly ticks on the x-axis?

You want to plot data with monthly ticks labeled as abbreviated month names (e.g., Jan, Feb). Which code snippet produces this correctly formatted x-axis?

A

ax.xaxis.set_major_locator(mdates.MonthLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))

B

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

C

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

D

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

💡 Hint

MonthLocator controls tick positions by month, DateFormatter controls label format.

MonthLocator places ticks at the start of each month. DateFormatter with '%b' formats labels as abbreviated month names like 'Jan', 'Feb'.

🔧 Debug

advanced

What error does this code raise when plotting dates?

This code attempts to plot dates on the x-axis but raises an error. What is the error type?

Matplotlib

import matplotlib.pyplot as plt
from datetime import datetime

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

plt.plot(dates, values)
plt.show()

A TypeError: float() argument must be a string or a number, not 'datetime.date'

B No error, plot displays correctly

C ValueError: could not convert string to float: '2024-01-01'

D AttributeError: 'str' object has no attribute 'toordinal'

💡 Hint

Matplotlib expects datetime objects or numbers, not date strings.

Matplotlib cannot convert date strings directly to floats for plotting, causing a ValueError.

🚀 Application

expert

What is the value of 'num_ticks' after running this code?

This code sets up a plot with dates on the x-axis and custom tick intervals. What is the number of major ticks on the x-axis after running?

Matplotlib

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

start = datetime(2024, 1, 1)
end = datetime(2024, 3, 1)

fig, ax = plt.subplots()
ax.set_xlim(start, end)
ax.xaxis.set_major_locator(mdates.MonthLocator(bymonthday=15))

# Count major ticks
num_ticks = len(ax.get_xticks())
plt.close(fig)

A 1

B 2

C 3

D 0

💡 Hint

MonthLocator with bymonthday=15 places ticks on the 15th of each month within limits.

Between 2024-01-01 and 2024-03-01, the 15th of January and 15th of February fall inside the range, so 2 ticks appear.