【Python】matplotlib 双y轴绘制及合并图例

1.双y轴绘制 关键函数：twinx()

　　问题在于此时图例会有两个。

 # -*- coding: utf-8 -*-
 import numpy as np
 import matplotlib.pyplot as plt
 from matplotlib import rc
 rc('mathtext', default='regular')

 time = np.arange(10)
 temp = np.random.random(10)*30
 Swdown = np.random.random(10)*100-10
 Rn = np.random.random(10)*100-10

 fig = plt.figure()
 ax = fig.add_subplot(111)
 ax.plot(time, Swdown, '-', label = 'Swdown')
 ax.plot(time, Rn, '-', label = 'Rn')
 ax2 = ax.twinx()
 ax2.plot(time, temp, '-r', label = 'temp')
 ax.legend(loc=0)
 ax.grid()
 ax.set_xlabel("Time (h)")
 ax.set_ylabel(r"Radiation ($MJ\,m^{-2}\,d^{-1}$)")
 ax2.set_ylabel(r"Temperature ($^\circ$C)")
 ax2.set_ylim(0, 35)
 ax.set_ylim(-20,100)
 ax2.legend(loc=0)
 plt.savefig('0.png')

　　每个句柄对应一个图例。

2. 合并图例

　　1） 仅使用一个轴的legend()函数

 # -*- coding: utf-8 -*-
 import numpy as np
 import matplotlib.pyplot as plt
 from matplotlib import rc
 rc('mathtext', default='regular')

 time = np.arange(10)
 temp = np.random.random(10)*30
 Swdown = np.random.random(10)*100-10
 Rn = np.random.random(10)*100-10

 fig = plt.figure()
 ax = fig.add_subplot(111)

 lns1 = ax.plot(time, Swdown, '-', label = 'Swdown')
 lns2 = ax.plot(time, Rn, '-', label = 'Rn')
 ax2 = ax.twinx()
 lns3 = ax2.plot(time, temp, '-r', label = 'temp')

 # added these three lines
 lns = lns1+lns2+lns3
 labs = [l.get_label() for l in lns]
 ax.legend(lns, labs, loc=0)

 ax.grid()
 ax.set_xlabel("Time (h)")
 ax.set_ylabel(r"Radiation ($MJ\,m^{-2}\,d^{-1}$)")
 ax2.set_ylabel(r"Temperature ($^\circ$C)")
 ax2.set_ylim(0, 35)
 ax.set_ylim(-20,100)
 plt.savefig('0.png')

　　可以看到y1轴和y2轴的图例已经合并了

　　2）使用figure.legend()

 # -*- coding: utf-8 -*-
 import numpy as np
 import matplotlib.pyplot as plt

 x = np.linspace(0,10)
 y = np.linspace(0,10)
 z = np.sin(x/3)**2*98

 fig = plt.figure()
 ax = fig.add_subplot(111)
 ax.plot(x,y, '-', label = 'Quantity 1')

 ax2 = ax.twinx()
 ax2.plot(x,z, '-r', label = 'Quantity 2')
 fig.legend(loc=1)

 ax.set_xlabel("x [units]")
 ax.set_ylabel(r"Quantity 1")
 ax2.set_ylabel(r"Quantity 2")

 plt.savefig('0.png')

　　可以看到图例位置不对，已经出界，需要使用bbox_to_anchor和bbox_transform设置。

　　fig.legend(loc=1, bbox_to_anchor=(1,1), bbox_transform=ax.transAxes)

 # -*- coding: utf-8 -*-
 import numpy as np
 import matplotlib.pyplot as plt

 x = np.linspace(0,10)
 y = np.linspace(0,10)
 z = np.sin(x/3)**2*98

 fig = plt.figure()
 ax = fig.add_subplot(111)
 ax.plot(x,y, '-', label = 'Quantity 1')

 ax2 = ax.twinx()
 ax2.plot(x,z, '-r', label = 'Quantity 2')
 fig.legend(loc=1, bbox_to_anchor=(1,1), bbox_transform=ax.transAxes)

 ax.set_xlabel("x [units]")
 ax.set_ylabel(r"Quantity 1")
 ax2.set_ylabel(r"Quantity 2")

 plt.savefig('0.png')

　　　可以看到图例已经正常了。

转自：StackOverflow