你需要从每个Axes中获取图例handles和labels，然后将所有句柄和标签的列表传递给图例。你可以使用ax.get_legend_handles_labels()来做到这一点：

import matplotlib.pyplot as plt
import pandas as pd

# Some sample data
df = pd.DataFrame({'C' : [4,5,6,7], 'S' : [10,20,30,40],'R' : [100,50,-30,-50]})

fig=plt.figure()
plt.title('Title')

ax1 = df.R.plot(style='b', label='Suc. Rate')
ax1.set_ylabel('Success Rate / Coherence')

ax2 = df.C.plot(style='r', label='Coherence')

ax3 = df.S.plot(secondary_y=True, style='g', label='Size')
ax3.set_ylabel('Lexicon Size')

handles,labels = [],[]
for ax in fig.axes:
    for h,l in zip(*ax.get_legend_handles_labels()):
        handles.append(h)
        labels.append(l)

plt.legend(handles,labels)

plt.show()