def _compute(self):
        """Compute r min max and labels position"""
        delta = 2 * pi / self._len if self._len else 0
        self._x_pos = [.5 * pi + i * delta for i in range(self._len + 1)]
        for serie in self.all_series:
            serie.points = [(v, self._x_pos[i])
                            for i, v in enumerate(serie.values)]
            if self.interpolate:
                extended_x_pos = ([.5 * pi - delta] + self._x_pos)
                extended_vals = (serie.values[-1:] + serie.values)
                serie.interpolated = list(
                    map(
                        tuple,
                        map(
                            reversed,
                            self._interpolate(extended_x_pos, extended_vals)
                        )
                    )
                )

        # x labels space
        self._box.margin *= 2
        self._rmin = self.zero
        self._rmax = self._max or 1
        self._box.set_polar_box(self._rmin, self._rmax)
        self._self_close = True
Compute r min max and labels position