"""
Shoiw how to display two scales on the left and right y axis -- Fahrenheit and Celcius
"""

import matplotlib.pyplot as plt

fig = plt.figure()
ax1 = fig.add_subplot(111)     # the Fahrenheit scale
ax2 = ax1.twinx()              # the Celcius scale

def Tc(Tf):
    return (5./9.)*(Tf-32)


def update_ax2(ax1):
   y1, y2 = ax1.get_ylim()
   ax2.set_ylim(Tc(y1), Tc(y2))
   ax2.figure.canvas.draw()

# automatically update ylim of ax2 when ylim of ax1 changes.
ax1.callbacks.connect("ylim_changed", update_ax2)
ax1.plot([78, 79, 79, 77])

ax1.set_title('Two scales: Fahrenheit and Celcius')
ax1.set_ylabel('Fahrenheit')
ax2.set_ylabel('Celcius')

plt.show()