title: pyqtgraph : how to plot time series (date and time on the x axis)?
body: <p>I want to plot time series with pyqtgraph, and display the date and/or time on the x axis scale, but I couldn't find how to do it.</p>
  
  <p>Edit 1 :
  It seems like I should subclass AxisItem and reimplement tickStrings(). I'll look into this.</p>
  
  <p>Edit 2 :
  Here is how I subclassed AxisItem. The documentation shows how to use the class.</p>
  
  <pre><code>from __future__ import print_function
  from PySide.QtCore import *
  from PySide.QtUiTools import *
  from PySide.QtGui import *
  import pyqtgraph as pg
  import time
  
  ## Reimplements \c pyqtgraph.AxisItem to display time series.
  # \code
  # from caxistime import CAxisTime
  # \# class definition here...
  # self.__axisTime=CAxisTime(orientation='bottom')
  # self.__plot=self.__glyPlot.addPlot(axisItems={'bottom': self.__axisTime}) # __plot : PlotItem
  # \endcode
  class CAxisTime(pg.AxisItem):
      ## Formats axis label to human readable time.
      # @param[in] values List of \c time_t.
      # @param[in] scale Not used.
      # @param[in] spacing Not used.
      def tickStrings(self, values, scale, spacing):
          strns = []
          for x in values:
              try:
                  strns.append(time.strftime("%H:%M:%S", time.gmtime(x)))    # time_t --&gt; time.struct_time
              except ValueError:  # Windows can't handle dates before 1970
                  strns.append('')
          return strns
  </code></pre>