Using DateFormatter resets starting date to 1970

Question

I have a dataframe where the index is the first date of each month and the size column is the frequency for that month, e.g.

Using .index on the dataframe confirms the type of the index is DatetimeIndex:

DatetimeIndex(['2006-12-01', ...],
              dtype='datetime64[ns]', name='created_at_month', length=175, freq=None)

Using .plot() on the DataFrame I can produce a line graph per month:

However, it only lists every other year on the x axis, and I'd like it to list each year on the axis.

I would expect to be able to do

ax.xaxis.set_major_locator(mdates.YearLocator(1))
ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y'))

However this doesn't output any labels at all.

If I add a minor formatter (ax.xaxis.set_minor_formatter(mdates.DateFormatter('%d %m %Y'))), I get this:

What am I doing wrong here to cause the dates to change?

The relevant versions are:

Matplotlib: 3.3.4
Pandas: 1.2.4
Python: 3.8.8

Lemme guess you're trying to display all years on the X axis? — The Singularity, Sep 08 '21 at 10:27
Yeah I want all the years labelled as major ticks, and then ideally all the months as minor ticks but I want to get years working first — Freddy Heppell, Sep 08 '21 at 10:29
See https://matplotlib.org/2.0.2/examples/api/date_demo.html — Trenton McKinney, Sep 08 '21 at 11:18
Here is a minimal example, I'll check if the first answer resolves this issue https://colab.research.google.com/drive/1U7jySEmK03SDPtC5Z1G9wRY7SBxAiXUN?usp=sharing — Freddy Heppell, Sep 08 '21 at 11:25

score 5 · Accepted Answer · answered Sep 08 '21 at 11:24

As reported here, for some reason pandas's plot shows this issue.
You can overcome this issue by replacing pandas' plot with matplotlib.pyplot.plot.
You can take this answer as a reference for 2 datetime ticks on x axis (month and year, or month and day, or day and hour, as you need), using two different axis in palce of minor and major ticks.

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


df = pd.DataFrame({'created_at_month': pd.date_range(start = '2006-12-01', end = '2020-12-01', freq = 'MS')})
df['size'] = np.random.randint(0, 200, len(df))
df = df.set_index('created_at_month')


fig, ax = plt.subplots()

ax.plot(df.index, df['size'])

ax.xaxis.set_major_locator(mdates.YearLocator(base = 1, month = 1, day = 1))
ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y'))

plt.show()

How is this still a problem 2 years later? – Clark McCauley Feb 25 '23 at 20:13 — Clark McCauley, Feb 25 '23 at 20:13

Using DateFormatter resets starting date to 1970

1 Answers1

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