Вот код для начала и эксперимента:
import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
dates = pd.date_range('20190822', '20191020', freq='D')
df3 = pd.DataFrame({'Date': dates,
'Temp.AM': np.random.normal(22.5, 2, len(dates)),
'Temp.PM': np.random.normal(27.5, 2, len(dates))})
df3.plot(x='Date', y=['Temp.PM', 'Temp.AM'], color=['dodgerblue','coral'], figsize=(20, 8))
min_temp = df3['Temp.AM'].min()
max_temp = df3['Temp.PM'].max()
plt.axhline(min_temp, c='coral', ls='--')
plt.axhline(max_temp, c='dodgerblue', ls='--')
plt.text(df3['Date'][0], min_temp, f'\nMin: {min_temp:.1f}°C', ha='left', va='center', c='coral')
plt.text(df3['Date'][0], max_temp, f'Max: {max_temp:.1f}°C\n', ha='left', va='center', c='dodgerblue')
plt.show()