title: time format of xaxis does not change correctly on pandas' plot
body: <p>I'd like to plot pandas DataFrame as a line chart with x-axis having '%H:%M' style.</p>
  
  <p>However, when I try this:</p>
  
  <pre><code>import datetime
  import pandas as pd
  import numpy as np
  d = pd.DataFrame([np.random.rand(3) for _ in range(9)])
  d.index = [datetime.time(i, 0) for i in range(15,24)]
  ax = d.plot(xticks=d.index)
  plt.xticks([x.strftime("%H:%M") for x in d.index][0::4])
  </code></pre>
  
  <p>Chart with %H:%M:%s style x-axis is ploted:</p>
  
  <p><a href="https://i.stack.imgur.com/COWQT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/COWQT.png" alt="enter image description here"></a></p>
  
  <p>Following this <a href="https://stackoverflow.com/questions/14946371/editing-the-date-formatting-of-x-axis-tick-labels-in-matplotlib">answer</a>, I've added <code>DateFormatter</code>:</p>
  
  <pre><code>import matplotlib.dates as md
  d = pd.DataFrame([np.random.rand(3) for _ in range(9)])
  d.index = [datetime.time(i, 0) for i in range(15,24)]
  ax = d.plot(xticks=d.index)
  #plt.xticks([x.strftime("%H:%M") for x in d.index][0::4])
  ax.xaxis.set_major_formatter(md.DateFormatter('%H:%M'))
  </code></pre>
  
  <p>Unfortunately, I got a plot with only 00:00 x-axis.</p>
  
  <p><a href="https://i.stack.imgur.com/dXO1e.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/dXO1e.png" alt="enter image description here"></a></p>
  
  <p>I'd like to get %H:%M style x-axis with proper interval ticks label like "15:00, 17:00, 19:00, 21:00" (every 2 intervals here). How do I realize that?</p>