title: ValueError: 'DateFormatter found a value of x=0, which is an illegal date'
body: <p>I have lists of time in seconds. e.g.: </p>
  
  <pre><code>[0, 15, 30, .... 18615, 18630, 18645]
  </code></pre>
  
  <p>and I want to plot it as hour:min:sec against an equally long list of values.</p>
  
  <p>I have tried using date formatter but I get the following error:</p>
  
  <pre><code>ValueError: DateFormatter found a value of x=0, which is an illegal date.  This usually occurs because you have not informed the axis that it is plotting dates, e.g., with ax.xaxis_date()
  </code></pre>
  
  <p>I have already implemented the <code>ax.xaxis_date()</code> so that shouldn't be the problem. Hope you can help me get the x-axis to show time in hours instead of seconds.
  The relevant code:</p>
  
  <pre><code>self.samp_freq = 15             
  self.noise_samp = np.arange(0, self.time_total, self.samp_freq)
  self.random_noise = np.random.normal(0, 10, len(self.noise_samp))
  
  self.noi_ax.xaxis.set_major_locator(mticker.MaxNLocator(5))
  self.noi_ax.xaxis_date()
  self.noi_ax.xaxis.set_major_formatter(mdates.DateFormatter('%H:%M:%S'))
  
  self.noi_ax.plot(self.noise_samp, self.scaled_random_noise)[0]
  self.noi_ax.set_xlabel('time (sec)')
  self.noi_ax.set_ylabel('noise (uGal')
  </code></pre>