def dx(self):
        """X-axis sample separation

        :type: `~astropy.units.Quantity` scalar
        """
        try:
            return self._dx
        except AttributeError:
            try:
                self._xindex
            except AttributeError:
                self._dx = Quantity(1, self.xunit)
            else:
                if not self.xindex.regular:
                    raise AttributeError("This series has an irregular x-axis "
                                         "index, so 'dx' is not well defined")
                self._dx = self.xindex[1] - self.xindex[0]
            return self._dx
X-axis sample separation

        :type: `~astropy.units.Quantity` scalar