def index_by(self, column_or_label):
column = self._get_column(column_or_label)<EOL>index = {}<EOL>for key, row in zip(column, self.rows):<EOL><INDENT>index.setdefault(key, []).append(row)<EOL><DEDENT>return index<EOL>
Return a dict keyed by values in a column that contains lists of
        rows corresponding to each value.