pyechart有关图片的创建

Question

c = (
    PictorialBar()
    .add_xaxis(list(dic.keys()))
    .add_yaxis(
        "2020",
        [
            {"value": 18, "symbol": dic['飞机']},
            {"value": 40, "symbol": dic['火车']},
            {"value": 78, "symbol": dic['汽车']},
            {"value": 66, "symbol": dic['轮船']},
        ],
        label_opts=opts.LabelOpts(is_show=False),
        symbol_size=22,
        symbol_repeat="fixed",
        symbol_offset=[0, 5],
        is_symbol_clip=True,
    )
    .add_yaxis(
        "2021",
        [
            {"value": 54, "symbol": dic['飞机']},
            {"value": 68, "symbol": dic['火车']},
            {"value": 105, "symbol": dic['汽车']},
            {"value": 73, "symbol": dic['轮船']},
        ],
        label_opts=opts.LabelOpts(is_show=False),
        symbol_size=22,
        symbol_repeat="fixed",
        symbol_offset=[0, -25],
        is_symbol_clip=True,
    )
    .reversal_axis()
    .set_global_opts(
        title_opts=opts.TitleOpts(title="城市交通工具"),
        xaxis_opts=opts.AxisOpts(is_show=False),
        yaxis_opts=opts.AxisOpts(
            axistick_opts=opts.AxisTickOpts(is_show=False),
            axisline_opts=opts.AxisLineOpts(
                linestyle_opts=opts.LineStyleOpts(opacity=0)
            ),
        ),
    )
)
c.render_notebook()

问题在于这个"symbol"后面访问的字典:
字典内容是：

我想要明白，这个字典的key所对应的value是需要符合什么格式？才能保证后续的symbol能正常导入图片