docs: def __date_format(self, x):
          """ formatter for date x-data. primitive, and probably needs
          improvement, following matplotlib's date methods.
          """
  
          if x < 1: x = 1
  
          span = self.axes.xaxis.get_view_interval()
          tmin = max(1.0, span[0])
          tmax = max(2.0, span[1])
          tmin = time.mktime(dates.num2date(tmin).timetuple())
          tmax = time.mktime(dates.num2date(tmax).timetuple())
          nhours = (tmax - tmin)/3600.0
          fmt = "%m/%d"
          if nhours < 0.1:
              fmt = "%H:%M\n%Ssec"
          elif nhours < 4:
              fmt = "%m/%d\n%H:%M"
          elif nhours < 24*8:
              fmt = "%m/%d\n%H:%M"
          try:
              return time.strftime(fmt, dates.num2date(x).timetuple())
          except:
              return "?"