docs: def _weight_opacity(weight, weight_range):
      # type: (float, float) -> str
      """ Return opacity value for given weight as a string.
      """
      min_opacity = 0.8
      if np.isclose(weight, 0) and np.isclose(weight_range, 0):
          rel_weight = 0.0
      else:
          rel_weight = abs(weight) / weight_range
      return '{:.2f}'.format(min_opacity + (1 - min_opacity) * rel_weight)