"""
Demo of how to display two scales on the left and right y axis.

This example uses the Fahrenheit and Celsius scales.
"""
import matplotlib.pyplot as plt
import numpy as np


def fahrenheit2celsius(temp):
    """
    Returns temperature in Celsius.
    """
    return (5. / 9.) * (temp - 32)


def convert_ax_c_to_celsius(ax_f):
    """
    Update second axis according with first axis.
    """
    y1, y2 = ax_f.get_ylim()
    ax_c.set_ylim(fahrenheit2celsius(y1), fahrenheit2celsius(y2))
    ax_c.figure.canvas.draw()

fig, ax_f = plt.subplots()
ax_c = ax_f.twinx()

# automatically update ylim of ax2 when ylim of ax1 changes.
ax_f.callbacks.connect("ylim_changed", convert_ax_c_to_celsius)
ax_f.plot(np.linspace(-40, 120, 100))
ax_f.set_xlim(0, 100)

ax_f.set_title('Two scales: Fahrenheit and Celsius')
ax_f.set_ylabel('Fahrenheit')
ax_c.set_ylabel('Celsius')

plt.show()
