Faceting creates new TableFu instances with rows matching
        each possible value.

def facet_by(self, column):
        """
        Faceting creates new TableFu instances with rows matching
        each possible value.
        """
        faceted_spreadsheets = {}
        for row in self.rows:
            if row[column]:
                col = row[column].value
                if faceted_spreadsheets.has_key(col):
                    faceted_spreadsheets[col].append(row.cells)
                else:
                    faceted_spreadsheets[col] = []
                    faceted_spreadsheets[col].append(row.cells)

        # create a new TableFu instance for each facet
        tables = []
        for k, v in faceted_spreadsheets.items():
            v.insert(0, self.default_columns)
            table = TableFu(v)
            table.faceted_on = k
            table.formatting = self.formatting
            table.options = self.options
            tables.append(table)

        tables.sort(key=lambda t: t.faceted_on)
        return tables