plot-x-y.py

# coding: utf-8

# http://www.omz-software.de/pythonista/docs/matplotlib/examples/api/fahrenheit_celsius_scales.html

# https://forum.omz-software.com/topic/2965/matplotlib-twins-does-not-function

"""
Show how to display two scales on the left and right y axis -- Fahrenheit and Celsius
"""

import matplotlib.pyplot as plt

fig, ax1 = plt.subplots()  # ax1 is the Fahrenheit scale
ax2 = ax1.twinx()          # ax2 is the Celsius 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 Celsius')
ax1.set_ylabel('Fahrenheit')
ax2.set_ylabel('Celsius')

plt.show()

# Resolved:
# ax2 = ax1.twinx()
# ax2.set_ylim(ax.get_ylim())