reindex_dfs() rebuilt the combined index via sorted(set(...)) +
pd.to_datetime, dropping the index name set by history.py ('Date' for
daily, 'Datetime' for intraday). After concat, data.index.name was
None, so users doing data.stack().reset_index() got a 'level_0' column
instead of 'Date' / 'Datetime'.
Use Index.union, which preserves the name natively when sources agree.
Drive-by: narrow data.columns to MultiIndex before swaplevel to silence
a pyright attribute warning.
Alessandro Colace
c3db565d09