绘制PandasDatafarme列时，是否可以使用Dataframe列名作为图例标签，而不是显式指定标签？
示例：

import matplotlib.pyplot as plt
import pandas as pd
df = pd.DataFrame(data={'col1': [0, 2, 1, 3], 
                        'col2': [9,7,8,9]}, 
                        index=[0, 1, 2, 3])

f = plt.figure()
ax = f.subplots()
ax.plot(df['col1'], label='col1') # How to not explicitly specify label?
# ax.plot(df['col1']) # This does not produce a legend label
ax.legend()