def _get_field_name(svl_axis):
    """ Extracts the name of the field for the SVL plot.

    Parameters
    ----------
    svl_axis : dict
        The SVL axis specifier.

    Returns
    -------
    str
        The name of the field in the axis, or the transform statement.
    """
    if "transform" in svl_axis:
        return svl_axis["transform"]
    elif "field" in svl_axis:
        return svl_axis["field"]
    else:
        return "*"