title: timestamps passed to matplotlib.date2num: 'str' object has no attribute 'toordinal'
body: <p>Have an array with timestamps (format <code>%Y-%M-%D %H:%M:%S</code>) collected from a textfile. I want to plot these in a subplot with matplotlib. But I can't get it to work. I was thinking of this:</p>
  
  <pre><code>import numpy as np
  import datetime
  import matplotlib.pyplot as plt
  import matplotlib.dates as md
  
  dateconv = lambda s: datetime.strptime(s, '%Y-%M-%D %H:%M:%S:.%f')
  
  col_names = ["timestamp", "light", "sensor1", "sensor2", "sensor3", "temp"]
  dtypes = ["object", "uint8", "uint8", "uint8", "uint8", "float"]
  mydata = np.genfromtxt("data.csv", delimiter=",", names = col_names, dtype=dtypes, converters={"Time": dateconv})
  
  
  time = md.date2num(mydata['timestamp'])
  sensor1 = mydata['sensor1']
  sensor2 = mydata['sensor2']
  sensor3 = mydata['sensor3']
  light = mydata['light']
  temp = mydata['temp']
  
  fig = plt.figure()
  rect = fig.patch
  rect.set_facecolor('#31312e')
  
  ax1 = fig.add_subplot(3,2,1, axisbg='grey')
  ax1.plot_date(time, sensor1, 'c', linewidth=2)
  ax1.tick_params(axis='x', colors='c')
  ax1.tick_params(axis='y', colors='c')
  ax1.spines['bottom'].set_color('w')
  ax1.spines['top'].set_color('w')
  ax1.spines['left'].set_color('w')
  ax1.spines['right'].set_color('w')
  ax1.yaxis.label.set_color('c')
  ax1.xaxis.label.set_color('c')
  ax1.set_title('Sensor 1', color = 'c')
  ax1.set_xlabel('Time')
  ax1.set_ylabel('Value')
  ax1.set_ylim(0, 255)
  
  ax2 = fig.add_subplot(3,2,2, axisbg='grey')
  #so on...
  
  plt.setp(ax1.xaxis.get_majorticklabels(), rotation = 25)
  plt.show()
  </code></pre>
  
  <p>But it is not working I get the following error:
  <code>'str' object has no attribute 'toordinal'</code> at line 18 (line with <code>md.date2num(mydata['timestamp'</code>)</p>
  
  <p>Data sample:</p>
  
  <pre><code>2014-08-12 22:45:12.826871, 65, 244, 213, 196, 21.625
  2014-08-12 22:50:14.151601, 66, 246, 208, 196, 21.312
  2014-08-12 22:55:15.399692, 15, 247, 208, 196, 21.375
  2014-08-12 23:00:16.717546, 15, 248, 209, 195, 21.5
  2014-08-12 23:05:18.041433, 15, 249, 212, 195, 21.625
  2014-08-12 23:10:19.372733, 16, 248, 216, 195, 21.687
  </code></pre>