def _sline_bokeh(self, window_size, y_label):
        """
        Returns a chart with a smooth line from a serie
        """
        try:
            ds2 = self.clone_()
            window = np.ones(int(window_size)) / float(window_size)
            ds2.df[y_label] = np.convolve(self.df[self.y], window, 'same')
            ds2.chart(self.x, y_label)
            return ds2.line_()
        except Exception as e:
            self.err(e, self._sline_bokeh, "Can not draw smooth line chart")