# sometimes it is nice to have ticklabels centered.  mpl currently
# associates a label with a tick, and the label can be aligned
# 'center', 'feft', or 'right' using the horizontal alignment property:
#
#
#   for label in ax.xaxis.get_xticklabels():
#       label.set_horizntal_alignment('right')
#
#
# but this doesn't help center the label between ticks.  One solution
# is to "face it".  Use the minor ticks to place a tick centered
# between the major ticks.  Here is an example that labels the months,
# centered between the ticks

import datetime
import numpy as np
import matplotlib
import matplotlib.dates as dates
import matplotlib.ticker as ticker
import matplotlib.pyplot as plt

# load some financial data; apple's stock price
fh = matplotlib.get_example_data('aapl.npy')
r = np.load(fh); fh.close()
r = r[-250:]  # get the last 250 days

fig = plt.figure()
ax = fig.add_subplot(111)
ax.plot(r.date, r.adj_close)

ax.xaxis.set_major_locator(dates.MonthLocator())
ax.xaxis.set_minor_locator(dates.MonthLocator(bymonthday=15))

ax.xaxis.set_major_formatter(ticker.NullFormatter())
ax.xaxis.set_minor_formatter(dates.DateFormatter('%b'))

for tick in ax.xaxis.get_minor_ticks():
    tick.tick1line.set_markersize(0)
    tick.tick2line.set_markersize(0)
    tick.label1.set_horizontalalignment('center')

imid = len(r)/2
ax.set_xlabel(str(r.date[imid].year))
plt.show()