Problems with datetime plot in matplotlib

Question

I am trying to plot some datetime data but I have two problems I can't solve. My code is the following:

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
from datetime import datetime as DT
import numpy as np

%matplotlib inline

dt = np.dtype([('date', 'datetime64[D]'), ('count', np.uint32)])
data = np.array([(np.datetime64('2014-12-' + str(i)), i) for i in range(11,20)], dtype=dt)

fig, ax = plt.subplots(figsize=(18,8), dpi=300)
ax.plot_date(data['date'], data['count'])
#ax.set_xlim( [mdates.date2num( (data['date'][0] - np.timedelta64(1, 'D')).astype(DT) ),
               mdates.date2num( (data['date'][-1] + np.timedelta64(1, 'D')).astype(DT) )]);

With the last line commented out I get a plot but with the x labels shifted to the right of 1 day.
When I try to set the x limits to the day before the first day of the data and to the day after the last day of the data the limits are correctly setted but I lose the data points

Your first plot looks incorrect. Why does it have dec 10 45 in the x-axis for example? Why the 45? Are you sure your data[date] is the correct date? — Ffisegydd
– Ffisegydd, Commented Dec 12, 2014 at 14:06
@Ffisegydd mmm... maybe you are right, in my actual file I am formatting the labels without showing the year so I didn't notice the '45'... but why if I initialize a date with np.datetime64('2014-12-10') it changes the year? — Red
– Red, Commented Dec 12, 2014 at 14:11

J Richard Snape · Accepted Answer · 2014-12-12 15:39:27Z

2

I replicated the behaviour that you see and it appears that matplotlib does not recognise the numpy datetime64 type.

This works - converting the np.datetime64 to a standard python datetime object:

import datetime
ax.plot_date([d.astype(datetime.datetime) for d in data['date']], data['count'])

edited Dec 12, 2014 at 15:39

answered Dec 12, 2014 at 15:14

J Richard Snape

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