How does raw_to_Xy() know which column to use to calculate returns

The documentation says

> computation of returns (if use_log then logarithmic else simple) - the first timestep is automatically deleted

How does this function know which column is the closing price and therefore which column to use to calculate returns

Start by reading the raw_to_Xy() implementation and the documentation passage describing return computation. Determine how the closing-price column is selected, then clarify that behavior in the documentation so users can identify which column is used.

Written by the indexing model from the issue text.