code: def get_extents(self, view, ranges, range_type='combined'):
          
          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)
docs: Supply custom, static extents because radial heatmaps always have
          the same boundaries.