code: def add_missing_facets(
      data: pd.DataFrame,
      layout: pd.DataFrame,
      vars: list[str],
      facet_vals: pd.DataFrame,
  ) -> tuple[pd.DataFrame, pd.DataFrame]:
      """
      Add missing facets
      """
      # When in a dataframe some layer does not have all
      # the facet variables, add the missing facet variables
      # and create new data where the points(duplicates) are
      # present in all the facets
      missing_facets = list(
          set(vars) - set(facet_vals.columns.tolist())
      )
      if missing_facets:
          to_add = layout.loc[:, missing_facets].drop_duplicates()
          to_add.reset_index(drop=True, inplace=True)
  
          # a point for each facet, [0, 1, ..., n-1, 0, 1, ..., n-1, ...]
          data_rep = np.tile(np.arange(len(data)), len(to_add))
          # a facet for each point, [0, 0, 0, 1, 1, 1, ... n-1, n-1, n-1]
          facet_rep = np.repeat(np.arange(len(to_add)), len(data))
  
          data = data.iloc[data_rep, :].reset_index(drop=True)
          facet_vals = facet_vals.iloc[data_rep, :].reset_index(
              drop=True
          )
          to_add = to_add.iloc[facet_rep, :].reset_index(drop=True)
          facet_vals = pd.concat(
              [facet_vals, to_add], axis=1, ignore_index=False
          )
  
      return data, facet_vals
  
imports: from __future__ import annotations
  import itertools
  import types
  import typing
  from copy import copy, deepcopy
  import numpy
  import pandas
  import pandas.api.types
  from .._utils import cross_join, match
  from ..exceptions import PlotnineError
  from ..scales.scales import Scales
  from .strips import Strips
label_desc: Use image node_id 4 for calling a global function with example usage: add_missing_facets(data, layout, vars, facet_vals) and returns: data,  facet_vals