old_contents: from __future__ import print_function, division
  import matplotlib.pyplot as plt
  import matplotlib.dates as mdates
  import numpy as np
  
  _to_ordinalf_np_vectorized = np.vectorize(mdates._to_ordinalf)
  
  def plot_series(series, ax=None, label=None, date_format='%d/%m/%y %H:%M:%S', **kwargs):
      """Plot function for series which is about 5 times faster than
      pd.Series.plot().
  
      Parameters
      ----------
      ax : matplotlib Axes, optional
          If not provided then will generate our own axes.
  
      label : str, optional
          The label for the plotted line. The
          caller is responsible for enabling the legend.
  
      date_format : str, optional, default='%d/%m/%y %H:%M:%S'
      """
      if ax is None:
          ax = plt.gca()
      ax.xaxis.axis_date(tz=series.index.tzinfo)
      ax.xaxis.set_major_formatter(mdates.DateFormatter(date_format))
      x = _to_ordinalf_np_vectorized(series.index.to_pydatetime())
      ax.plot(x, series, label=label, **kwargs)
      ax.set_ylabel('watts')
      return ax
  
new_contents: from __future__ import print_function, division
  import matplotlib.pyplot as plt
  import matplotlib.dates as mdates
  import numpy as np
  
  _to_ordinalf_np_vectorized = np.vectorize(mdates._to_ordinalf)
  
  def plot_series(series, ax=None, label=None, date_format='%d/%m/%y %H:%M:%S', **kwargs):
      """Plot function for series which is about 5 times faster than
      pd.Series.plot().
  
      Parameters
      ----------
      ax : matplotlib Axes, optional
          If not provided then will generate our own axes.
  
      label : str, optional
          The label for the plotted line. The
          caller is responsible for enabling the legend.
  
      date_format : str, optional, default='%d/%m/%y %H:%M:%S'
      """
      if ax is None:
          fig, ax = plt.subplots(1)
      x = _to_ordinalf_np_vectorized(series.index.to_pydatetime())
      ax.plot(x, series, label=label, **kwargs)
      ax.xaxis.set_major_formatter(mdates.DateFormatter(date_format, 
                                                        tz=series.index.tzinfo))
      ax.set_ylabel('watts')
      fig.autofmt_xdate()
      plt.draw()
      return ax
  
subject: Fix bug where timezone was not used for xaxis.