title: Tick labels only displayed in one subplot
body: <p>Need to display custom shared x-axis tick labels on both subplots, using two datasets with different dates.</p>
  
  <pre><code>from pandas import DataFrame, date_range, Timedelta
  import numpy as np
  from matplotlib import pyplot as plt
  import matplotlib.dates as mdates
  
  #Dataset 1
  rng1 = date_range(start='01-01-2015', periods=5, freq='1M')
  df1 = DataFrame({'y':np.random.normal(size=len(rng1))}, index=rng1)
  y1 = df1['y']
  
  #Dataset 2
  rng2 = date_range(start='01-01-2015', periods=5, freq='2M')
  df2 = DataFrame({'y':np.random.normal(size=len(rng2))}, index=rng2)
  y2 = df2['y']
  
  #Figure
  fig,(ax1,ax2) = plt.subplots(2,1,sharex=True)
  
  y1.plot(ax=ax1)
  y2.plot(ax=ax2)
  plt.xticks(rotation=30)
  ax1.xaxis.set_minor_formatter(plt.NullFormatter())
  
  ax2.xaxis.set_major_locator(mdates.MonthLocator(interval=1))
  ax2.xaxis.set_major_formatter(mdates.DateFormatter('%D'))
  
  plt.show()
  </code></pre>
  
  <p>(Above) Creates figure without x-axis labels on the upper subplot</p>
  
  <p>I expect adding the below code to display the same x-axis labels to the upper subplot, but they are not showing up. What am I doing wrong? </p>
  
  <pre><code>ax1.xaxis.set_major_locator(mdates.MonthLocator(interval=1))
  ax1.xaxis.set_major_formatter(mdates.DateFormatter('%D'))
  </code></pre>
  
  <p>Setting sharex=False does not work because the dates are different for each dataset.</p>