How to Add Labels to the Legend in Plots with Secondary Axes Using twinx()?

Adding Labels to the Legend in Plots with Secondary Axes Using twinx()

Having multiple axes in a single plot can be useful for visualizing data from different sources or with different units. When using the twinx() function to create a secondary axis, it may be necessary to add labels to the lines plotted on the secondary axis and include them in the legend.

To accomplish this, you can either add a separate legend for the secondary axis using ax2.legend(loc=0). However, this approach results in two separate legends.

For a more cohesive display, all labels can be added to a single legend using the following steps:

• Create a list of line objects representing all lines plotted on both axes.
• Create a list of labels corresponding to the line objects.
• Use the legend function to add the lines and labels to the legend, passing the loc parameter to specify the legend's location.

<code class="python">import numpy as np
import matplotlib.pyplot as plt
from matplotlib import rc

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')

# Add all lines and labels to a single legend
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()</code>

This code will produce a single legend that includes all the labels from the primary and secondary axes.

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