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?

enter image description here

You can easily add a second legend by adding the line:

    ax2.legend(loc=0)

You'll get this:

enter image description here

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:

enter image description here

From: stackoverflow.com/q/5484922