def baseline(value):
        """ValueRef : string, vertical alignment of mark

        Possible values are ``'top'``, ``'middle'``, and ``'bottom'``. Only
        used if ``type`` is ``'image'`` or ``'text'``.
        """
        if value.value:
            _assert_is_type('shape.value', value.value, str_types)
            if value.value not in PropertySet._valid_baseline:
                raise ValueError(value.value + ' is not a valid baseline')