Retrieve validation plot data from grading YAML file (old style).

def _get_validate_plotdata_yaml(grading_file, data):
    """Retrieve validation plot data from grading YAML file (old style).
    """
    with open(grading_file) as in_handle:
        grade_stats = yaml.safe_load(in_handle)
    for sample_stats in grade_stats:
        sample = sample_stats["sample"]
        for vtype, cat, val in _flatten_grading(sample_stats):
            yield [sample, variant.get("variantcaller", ""),
                   vtype, cat, val]