How to plot realtime data with time x axis

Question

I need some help on how to plot realtime data (plt.draw()) with time on the x axis (hours minutes seconds) I have searched for two weeks and I just have been fighting bewildering errors. I finally have a partially working program but setting the xlims seems impossible. I also do not understand why the plot wont center on my data. Any advice on obtaining my goal and or solving my set_xlim error. I have tried every solution on this site and none of them have helped.

import matplotlib.pyplot as plt
import matplotlib.dates as md
import numpy as np
import datetime
import time
datenums=np.array((10), dtype = float)
n=1
duration=10
plt.ion()
plt.show() 
for n in range(1,5):
    now=time.mktime(time.localtime())
    timestamps=np.linspace(now,now+duration,n)
    dates=[datetime.datetime.fromtimestamp(ts) for ts in timestamps]
    datenums=md.date2num(dates)

    plt.subplots_adjust(bottom=0.2)
    plt.xticks(rotation = 45)
    x=datenums[n-1]
    ax=plt.gca() 
    ax.xaxis_date()
    xfmt = md.DateFormatter('%y-%m-%d %H:%M:%S')
    ax.xaxis.set_major_formatter(xfmt)
    print n,x
    time.sleep(1)
    ax.plot_date(x,n,markerfacecolor='CornflowerBlue')
    #ax.scatter(x,n)
    #ax.set_xlim(['2015-05-03 10:36:00', '2015-05-05 10:36:00'])

    plt.draw()

 plt.show()