попробуйте использовать plt.axvline () с matplotlib
import matplotlib.pyplot as plt
x = [ 2015, 2016, 2017, 2018,2019,2020]
y = [ 1000, 1200, 2500, 1000, 1100,250]
plt.plot(x,y)
plt.title("Sales Bar graph")
plt.xlabel("year")
plt.ylabel('Sales')
#drwa a line in 2019 value
plt.axvline(x=2019, label='line at x = {}'.format(2019), c='red')
plt.show()