Returns the dimension for the name of the trait for the specified mark.

    If `mark_type` is `None`, then the `trait_name` is returned
    as is.
    Returns `None` if the `trait_name` is not valid for `mark_type`.

def _get_attribute_dimension(trait_name, mark_type=None):
    """Returns the dimension for the name of the trait for the specified mark.

    If `mark_type` is `None`, then the `trait_name` is returned
    as is.
    Returns `None` if the `trait_name` is not valid for `mark_type`.
    """
    if(mark_type is None):
        return trait_name
    scale_metadata = mark_type.class_traits()['scales_metadata']\
        .default_args[0]
    return scale_metadata.get(trait_name, {}).get('dimension', None)