Supply custom, static extents because radial heatmaps always have
        the same boundaries.

def get_extents(self, view, ranges, range_type='combined'):
        """Supply custom, static extents because radial heatmaps always have
        the same boundaries.
        """
        if range_type not in ('data', 'combined'):
            return (None,)*4
        lower = -self.radius_outer
        upper = 2 * self.max_radius + self.radius_outer
        return (lower, lower, upper, upper)