Validate the value for the LinePlotter.plot formatoption

    Parameters
    ----------
    value: None, str or list with mixture of both
        The value to validate

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