Change tick frequency on X (time, not number) frequency

Whatever reason it is you are getting 2 ticks only by default, you can fix it (customise it) by changing the ticker locator using a date locator.

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

x = [ datetime.datetime(1900,1,1,0,1,2),
      datetime.datetime(1900,1,1,0,1,3),
      ...
      ]                                            # ( more than 1000 elements )
y = [ 34, 33, 23, ............ ]

fig = plt.figure()
ax = fig.add_subplot(1,1,1)  
plt.plot( x, y )

ax.xaxis.set_major_locator(mdates.MinuteLocator(interval=15))   #to get a tick every 15 minutes
ax.xaxis.set_major_formatter(mdates.DateFormatter('%H:%M'))     #optional formatting 

You have several locators (for example: DayLocator, WeekdayLocator, MonthLocator, etc.) read about it in the documentation:

http://matplotlib.org/api/dates_api.html

But maybe this example will help more:

http://matplotlib.org/examples/api/date_demo.html