Dates on x-axis in Matplotlib - Time & Space Complexity

Time Complexity: Dates on x-axis

O(n)

Understanding Time Complexity

When plotting dates on the x-axis using matplotlib, it's important to understand how the plotting time changes as the number of dates grows.

We want to know how the time to draw the plot increases when we add more date points.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

dates = [datetime.date(2023, 1, i+1) for i in range(100)]
values = list(range(100))

plt.figure()
plt.plot(dates, values)
plt.gca().xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))
plt.gcf().autofmt_xdate()
plt.show()

This code plots 100 dates on the x-axis with corresponding values, formatting the dates for better readability.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Plotting each date-value pair on the graph.
• How many times: Once for each date in the list (n times).

How Execution Grows With Input

As the number of dates increases, the plotting time grows roughly in direct proportion.

Input Size (n)	Approx. Operations
10	10 plot points processed
100	100 plot points processed
1000	1000 plot points processed

Pattern observation: Doubling the number of dates roughly doubles the work done to plot them.

Final Time Complexity

Time Complexity: O(n)

This means the time to plot grows linearly with the number of dates you want to show.

Common Mistake

[X] Wrong: "Plotting dates is always slow because date formatting is complex and slows everything down."

[OK] Correct: While formatting dates adds some work, the main time cost comes from plotting each point, which grows linearly. The formatting cost is small compared to plotting many points.

Interview Connect

Understanding how plotting time grows with data size helps you explain performance in data visualization tasks clearly and confidently.

Self-Check

What if we changed the x-axis to show only monthly ticks instead of daily dates? How would the time complexity change?