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Fixing common date annoyances
=============================
Matplotlib allows you to natively plots python datetime instances, and
for the most part does a good job picking tick locations and string
formats.  There are a couple of things it does not handle so
gracefully, and here are some tricks to help you work around them.
We'll load up some sample date data which contains datetime.date
objects in a numpy record array::
  In [63]: datafile = cbook.get_sample_data('goog.npz')
  In [64]: r = np.load(datafile)['price_data'].view(np.recarray)
  In [65]: r.dtype
  Out[65]: dtype([('date', '<M8[D]'), ('', '|V4'), ('open', '<f8'),
                  ('high', '<f8'), ('low', '<f8'), ('close', '<f8'),
                  ('volume', '<i8'),  ('adj_close', '<f8')])
  In [66]: r.date
  array(['2004-08-19', '2004-08-20', '2004-08-23', ..., '2008-10-10',
         '2008-10-13', '2008-10-14'], dtype='datetime64[D]')
The dtype of the NumPy record array for the field ``date`` is ``datetime64[D]``
which means it is a 64-bit `numpy.datetime64` in 'day' units. While this format
is more portable, Matplotlib cannot plot this format natively yet. We can plot
this data by changing the dates to `datetime.date` instances instead, which can
be achieved by converting to an object array::
  In [67]: r.date.astype('O')
  array([datetime.date(2004, 8, 19), datetime.date(2004, 8, 20),
         datetime.date(2004, 8, 23), ..., datetime.date(2008, 10, 10),
         datetime.date(2008, 10, 13), datetime.date(2008, 10, 14)],
        dtype=object)
The dtype of this converted array is now ``object`` and it is filled with
datetime.date instances instead.
If you plot the data, ::
  In [67]: plot(r.date.astype('O'), r.close)
  Out[67]: [<matplotlib.lines.Line2D object at 0x92a6b6c>]
you will see that the x tick labels are all squashed together.
import matplotlib.cbook as cbook
import matplotlib.dates as mdates
import numpy as np
import matplotlib.pyplot as plt
with cbook.get_sample_data('goog.npz') as datafile:
    r = np.load(datafile)['price_data'].view(np.recarray)
# Matplotlib prefers datetime instead of np.datetime64.
date = r.date.astype('O')
fig, ax = plt.subplots()
ax.plot(date, r.close)
ax.set_title('Default date handling can cause overlapping labels')
###############################################################################
# Another annoyance is that if you hover the mouse over the window and
# look in the lower right corner of the matplotlib toolbar
# (:ref:`navigation-toolbar`) at the x and y coordinates, you see that
# the x locations are formatted the same way the tick labels are, e.g.,
# "Dec 2004".
# What we'd like is for the location in the toolbar to have
# a higher degree of precision, e.g., giving us the exact date out mouse is
# hovering over.  To fix the first problem, we can use
# :func:`matplotlib.figure.Figure.autofmt_xdate` and to fix the second
# problem we can use the ``ax.fmt_xdata`` attribute which can be set to
# any function that takes a scalar and returns a string.  matplotlib has
# a number of date formatters built in, so we'll use one of those.
fig, ax = plt.subplots()
ax.plot(date, r.close)
# rotate and align the tick labels so they look better
fig.autofmt_xdate()
# use a more precise date string for the x axis locations in the
ax.fmt_xdata = mdates.DateFormatter('%Y-%m-%d')
ax.set_title('fig.autofmt_xdate fixes the labels')
###############################################################################
# Now when you hover your mouse over the plotted data, you'll see date
# format strings like 2004-12-01 in the toolbar.
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