title: pyqtgraph, plotting time series
body: <p>I'am trying to plot time serie with pyqtgraph. I've read  <a href="https://groups.google.com/forum/#!msg/pyqtgraph/pSIn0xNGodA/VsM9-3NJDZAJ" rel="nofollow noreferrer">this</a>, <a href="https://stackoverflow.com/questions/23151612/pyqtgraph-how-to-plot-time-series-date-and-time-on-the-x-axis">this</a> and <a href="https://gist.github.com/friendzis/4e98ebe2cf29c0c2c232" rel="nofollow noreferrer">this</a>. But i'am not sure how to correctly use it. </p>
  
  <p>My plot is a plot widget, and i use it this way:</p>
  
  <pre><code>graph.plot(aerosol_data, pen=pg.mkPen(color=colors[count], width=1, style=QtCore.Qt.SolidLine), axisItems={'bottom': TimeAxisItem(orientation='bottom')})
  </code></pre>
  
  <p>where TimeAxisItem is defined like this:</p>
  
  <pre><code>class TimeAxisItem(pg.AxisItem):
      def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
  
      def tickStrings(self, values, scale, spacing):
          # PySide's QTime() initialiser fails miserably and dismisses args/kwargs
          return [useful_values_dict['useful_data']['data']['ISO_dates']]
  </code></pre>
  
  <p>where ISO_dates is a list of date and time in ISO format</p>
  
  <p>I've also tried this:</p>
  
  <pre><code>graph.plotItem.plot(aerosol_data, pen=pg.mkPen(color=colors[count], width=1, style=QtCore.Qt.SolidLine), axisItems={'bottom': TimeAxisItem(orientation='bottom')})
  </code></pre>
  
  <p>but with no effects (axis strings are stil numbers).</p>
  
  <p>Then i tried to use the DateTimeAxis.py, this way:</p>
  
  <pre><code>date_axis = pg.DateAxisItem('bottom', pen=None, linkView=None, parent=None, maxTickLength=-1, showValues=True)
  date_axis.tickStrings(useful_values_dict['useful_data']['data']['timestamp_dates'],1, 1)
  </code></pre>
  
  <p>but i get an error:</p>
  
  <pre><code>File "C:\Python34\lib\site-packages\pyqtgraph\graphicsItems\DateAxisItem.py", line 161, in tickStrings
  format_strings.append(x.strftime(tick_spec.format))
  AttributeError: 'NoneType' object has no attribute 'format'
  </code></pre>