docs: def list_of_vars(arg_plot):
      """Construct list of variables per plot.
  
      Args:
          arg_plot (str): string with variable names separated with
              ``_`` (figures), ``.`` (subplots) and ``,`` (same subplot).
      Returns:
          three nested lists of str
  
          - variables on the same subplot;
          - subplots on the same figure;
          - figures.
      """
      lovs = [[[var for var in svars.split(',') if var]
               for svars in pvars.split('.') if svars]
              for pvars in arg_plot.split('-') if pvars]
      lovs = [[slov for slov in lov if slov] for lov in lovs if lov]
      return [lov for lov in lovs if lov]