from pathlib import Path

import matplotlib
import matplotlib.pyplot as plt
import numpy as np


def celsius_to_fahrenheit(celsius):
    return celsius * 9 / 5 + 32


def fahrenheit_to_celsius(fahrenheit):
    return (fahrenheit - 32) * 5 / 9


months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun"]
temperature_c = np.array([4, 7, 12, 17, 22, 26])

fig, ax = plt.subplots(figsize=(6, 4), layout="constrained")
ax.plot(months, temperature_c, marker="o", linewidth=2)
ax.set_title("Average monthly temperature")
ax.set_xlabel("Month")
ax.set_ylabel("Temperature (deg C)")
ax.set_ylim(0, 30)
ax.grid(True, axis="y", alpha=0.3)

secondary = ax.secondary_yaxis(
    "right", functions=(celsius_to_fahrenheit, fahrenheit_to_celsius)
)
secondary.set_ylabel("Temperature (deg F)")

output = Path("temperature-secondary-axis.png")
fig.savefig(output, dpi=160)
fig.canvas.draw()

check_c = np.array([0, 20, 30])
check_f = celsius_to_fahrenheit(check_c)
round_trip_c = fahrenheit_to_celsius(check_f)

print(f"matplotlib: {matplotlib.__version__} ({matplotlib.get_backend()} backend)")
print(f"primary y label: {ax.get_ylabel()}")
print(f"secondary y label: {secondary.get_ylabel()}")
print(f"conversion check: {check_c.tolist()} deg C -> {check_f.tolist()} deg F")
print(f"round trip: {round_trip_c.tolist()} deg C")
print(f"saved: {output}")
print(f"bytes: {output.stat().st_size}")

plt.close(fig)