[Python] Plotting timestamps (hour/minute/seconds) with Matplotlib

I want to plot some timestamps (Year-month-day Hour-Minute-Second format). I am using the following code, however it doesn't show any hour-minute-second information, it shows them as 00-00-00. I double checked my date array, and as you can see from the snippet below, they are not zero.

Do you have any idea about why I am getting 00-00-00's?

import matplotlib.pyplot as plt
import matplotlib.dates as md
import dateutil

dates = [dateutil.parser.parse(s) for s in datestrings]
# datestrings = ['2012-02-21 11:28:17.980000', '2012-02-21 12:15:32.453000', '2012-02-21 23:26:23.734000', '2012-02-26 17:42:15.804000']
plt.subplots_adjust(bottom=0.2)
plt.xticks( rotation= 80 )
ax=plt.gca()
xfmt = md.DateFormatter('%Y-%m-%d %H:%M:%S')
ax.xaxis.set_major_formatter(xfmt)
plt.plot(dates[0:10],plt_data[0:10], "o-")
plt.show()

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