def interpolate(self):
        """
        Interpolate across any missing zones.

        TODO
        Allow spline interpolation.
        """
        nans, x = utils.nan_idx(self)
        self[nans] = np.interp(x(nans), x(~nans), self[~nans])
        return self
Interpolate across any missing zones.

        TODO
        Allow spline interpolation.