图例从左到右填充在列中。换句话说，如果您诱使它认为还有另一行（图例中没有任何文本或线条颜色），则可以填充“图3”下的空间。
import numpy as np
import matplotlib.pyplot as plt
from pylab import *
X = np.linspace(0,100,11)
plt.plot(X,-X, label='plot 1', color='red')
plt.plot(X,-2*X, label='plot 2', color='green')
plt.plot(X,-3*X, label='plot 3', color='blue')
line1 = Line2D(range(10), range(10), marker='', color="red")
line2 = Line2D(range(10), range(10), marker='',color="green")
line3 = Line2D(range(10), range(10), marker='', color="blue")
line4 = Line2D(range(10), range(10), marker='', color="white")
plt.legend((line1,line4, line3,line2),('plot1','','plot3','plot2'),numpoints=1, loc=4,ncol=2)
plt.show()
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