title: Plotting of pandas DataFrame and xaxis as Timestamp produces empty plot
body: <p>I have a pandas.DataFrame (df), which consists of some values and a datetime which is a string at first but which I convert to a Timestamp using </p>
  
  <pre><code>df['datetime'] = pd.to_datetime(df['Time [dd.mm.yyyy hh:mm:ss.ms]'], format="%d.%m.%Y %H:%M:%S.%f")
  </code></pre>
  
  <p>It seems to work and I can access the new column's element's properties like obj.day and such. So the resulting column contains a Timestamp. When I try to plot this by using either <code>pyplot.plot(df['datetime'],df['value_name'])</code> or <code>df.plot(x='datetime',y='value_name')</code>,the picture below is the reslut. I tried converting the Timestamps using <code>obj.to_pydatetime()</code> but that did not change anything. The dataframe itself is populated by some data coming from csvs. What confuses me, is that with a certain csvs it works but with others not. I am pretty sure that the conversion to Timestamps was successful but I could be wrong. Also my time window should be from 2015-2016 not from 1981-1700. If I try to locate the min and max Timestamp from the DataFrame, I get the right Timestamps in 2015 and 2016 respectively.</p>
  
  <p><a href="https://i.stack.imgur.com/p98EO.png" rel="noreferrer">Resulting Picture form pyplot.plot</a></p>
  
  <p>Edit:
  <code>df.head()</code> gives:</p>
  
  <p>Sweep Time [dd.mm.yyyy hh:mm:ss.ms]  Frequency [Hz]  Voltage [V]<br>
  0    1.0       11.03.2014 10:13:04.270         50.0252      230.529<br>
  1    2.0       11.03.2014 10:13:06.254         49.9515      231.842<br>
  2    3.0       11.03.2014 10:13:08.254         49.9527      231.754<br>
  3    4.0       11.03.2014 10:13:10.254         49.9490      231.678<br>
  4    5.0       11.03.2014 10:13:12.254         49.9512      231.719   </p>
  
  <p>datetime<br>
  0 2014-03-11 10:13:04.270<br>
  1 2014-03-11 10:13:06.254<br>
  2 2014-03-11 10:13:08.254<br>
  3 2014-03-11 10:13:10.254<br>
  4 2014-03-11 10:13:12.254  </p>
  
  <p>and <code>df.info()</code> gives:</p>
  
  <pre><code>&lt;class 'pandas.core.frame.DataFrame'&gt;
  Int64Index: 33270741 entries, 0 to 9140687
  Data columns (total 5 columns):
  Sweep                            float64
  Time [dd.mm.yyyy hh:mm:ss.ms]    object
  Frequency [Hz]                   float64
  Voltage [V]                      float64
  datetime                         datetime64[ns]
  dtypes: datetime64[ns](1), float64(3), object(1)
  memory usage: 1.5+ GB
  </code></pre>
  
  <p>I am trying to plot 'Frequency [Hz]'vs 'datetime'.</p>