The Date column in stock_df is recorded as a series of Python strings. Although Seaborn can use string-formatted dates in some functions, Matplotlib cannot. To make the dates malleable to data processing and visualizations, we need to convert the values to float numbers supported by Matplotlib:

from matplotlib.dates import date2num


# Convert Date column from string to Python datetime object,
# then to float number that is supported by Matplotlib.
stock_df["Datetime"] = date2num(pd.to_datetime(stock_df["Date"], format="%Y-%m-%d").tolist())