docs: def validate_lineplot(value):
      """Validate the value for the LinePlotter.plot formatoption
  
      Parameters
      ----------
      value: None, str or list with mixture of both
          The value to validate"""
      if value is None:
          return value
      elif isinstance(value, six.string_types):
          return six.text_type(value)
      else:
          value = list(value)
          for i, v in enumerate(value):
              if v is None:
                  pass
              elif isinstance(v, six.string_types):
                  value[i] = six.text_type(v)
              else:
                  raise ValueError('Expected None or string, found %s' % (v, ))
      return value