import numpy as np

from pathlib import Path
import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
days = np.arange(1, 4)
litres = np.array([12, 18, 24])
target = Path("tank-readings.png")
if target.exists():
    raise FileExistsError("Choose an empty exercises folder")
fig, ax = plt.subplots(figsize=(8, 4.5), dpi=120)
ax.plot(days, litres, marker="o", color="#0078d4")
ax.set(xlabel="Observation day", ylabel="Collected water (litres)", title="Roof A: fictional collection readings", xticks=days)
fig.tight_layout()
fig.savefig(target)
plt.close(fig)
print("Saved tank-readings.png")