函数中有一个与图例对应的参数；默认情况下为True
df.plot(legend=False)
以下是
.plot()方法的定义
定义：df.plot（frame = None，x = None，y = None，subplots = False，sharex =
True，sharey = False，use_index = True，figsize = None，grid = None，legend =
True，rot = None， ax = None，style = None，title = None，xlim = None，ylim =
None，logx = False，logy = False，xticks = None，yticks = None，kind
=’line’，sort_columns = False，fontsize = None， secondary_y = False，** kwds）
