Called by a column when its width changes, triggering the graphic
        frame to recalculate its total width (as the sum of the column
        widths).

def notify_width_changed(self):
        """
        Called by a column when its width changes, triggering the graphic
        frame to recalculate its total width (as the sum of the column
        widths).
        """
        new_table_width = sum([col.width for col in self.columns])
        self._graphic_frame.width = new_table_width