def METHOD_NAME(self, stem_dict):
    """
    stem_dict: {45.5: 1, 47.0: 2}
    """
    # key: stem width
    # value: stem count
    width_dict = defaultdict(int)
    for width, count in stem_dict.items():
        width = self.round_value(width)
        width_dict[width] += count
    return width_dict