Secondary axis with twinx(): how to add to legend?
I have a plot with two y-axes, using twinx() . I also give labels to the lines, and want to show them with legend() , but I only succeed to get the labels of one axis in the legend:
import numpy as np
import matplotlib.pyplot as plt
from matplotlib import rc
rc('mathtext', default='regular')
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)
plt.show()
So I only get the labels of the first axis in the legend, and not the label 'temp' of the second axis. How could I add this third label to the legend?
You can easily add a second legend by adding the line:
ax2.legend(loc=0)
You'll get this:
But if you want all labels on one legend then you should do something like this:
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.show()
Which will give you this:
我不确定此功能是否是新功能，但您也可以使用get_legend_handles_labels（）方法，而不是跟踪自己的行和标签：
import numpy as np
import matplotlib.pyplot as plt
from matplotlib import rc
rc('mathtext', default='regular')
pi = np.pi
# fake data
time = np.linspace (0, 25, 50)
temp = 50 / np.sqrt (2 * pi * 3**2)
* np.exp (-((time - 13)**2 / (3**2))**2) + 15
Swdown = 400 / np.sqrt (2 * pi * 3**2) * np.exp (-((time - 13)**2 / (3**2))**2)
Rn = Swdown - 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')
# ask matplotlib for the plotted objects and their labels
lines, labels = ax.get_legend_handles_labels()
lines2, labels2 = ax2.get_legend_handles_labels()
ax2.legend(lines + lines2, labels + labels2, 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.show()
You can easily get what you want by adding the line in ax:
ax.plot(0, 0, '-r', label = 'temp')
or
ax.plot(np.nan, '-r', label = 'temp')
This would plot nothing but add a label to legend of ax.
I think this is a much easier way. It's not necessary to track lines automatically when you have only a few lines in the second axes, as fixing by hand like above would be quite easy. Anyway, it depends on what you need.
The whole code is as below:
import numpy as np
import matplotlib.pyplot as plt
from matplotlib import rc
rc('mathtext', default='regular')
time = np.arange(22.)
temp = 20*np.random.rand(22)
Swdown = 10*np.random.randn(22)+40
Rn = 40*np.random.rand(22)
fig = plt.figure()
ax = fig.add_subplot(111)
ax2 = ax.twinx()
#---------- look at below -----------
ax.plot(time, Swdown, '-', label = 'Swdown')
ax.plot(time, Rn, '-', label = 'Rn')
ax2.plot(time, temp, '-r') # The true line in ax2
ax.plot(np.nan, '-r', label = 'temp') # Make an agent in ax
ax.legend(loc=0)
#---------------done-----------------
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.show()
The plot is as below:
Update: add a better version:
ax.plot(np.nan, '-r', label = 'temp')
This will do nothing while plot(0, 0) may change the axis range.
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