formatter for date x-data. primitive, and probably needs
        improvement, following matplotlib's date methods.

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 "?"