Add sort_index() to closing data

[dan3dewey]

The dataframes do not seem to be in date-sorted order, this is not a problem for the plotting of the time series, but the autocorrelation, etc. assumes they are sorted.

A work-around is to add a sort_index() to the first "Munge the data" code cell, e.g., to read:

closing_data = pd.DataFrame()
   . . .
closing_data['aord_close'] = aord['Close']

# Put the closing_data in sorted order *** Needed for autocorrelation to work ***
closing_data = closing_data.sort_index()

# Pandas includes a very convenient function for filling gaps in the data.
closing_data = closing_data.fillna(method='ffill')