How to use the autovizwidget.autovizwidget.widget.encoding.Encoding.chart_type_line function in autovizwidget

def _create_viz_types_buttons(self):
        hbox = self.ipywidget_factory.get_hbox()
        children = list()

        if len(self.df) > 0:
            self.heading = self.ipywidget_factory.get_html('Type:', width='80px', height='32px')
            children.append(self.heading)

            self._create_type_button(Encoding.chart_type_table, children)
            self._create_type_button(Encoding.chart_type_pie, children)

            if len(self.df.columns) > 1:
                self._create_type_button(Encoding.chart_type_scatter, children)
                self._create_type_button(Encoding.chart_type_line, children)
                self._create_type_button(Encoding.chart_type_area, children)
                self._create_type_button(Encoding.chart_type_bar, children)

        hbox.children = children

        return hbox

def _get_graph(chart_type):
        if chart_type == Encoding.chart_type_scatter:
            graph = ScatterGraph()
        elif chart_type == Encoding.chart_type_line:
            graph = LineGraph()
        elif chart_type == Encoding.chart_type_area:
            graph = AreaGraph()
        elif chart_type == Encoding.chart_type_bar:
            graph = BarGraph()
        elif chart_type == Encoding.chart_type_pie:
            graph = PieGraph()
        elif chart_type == Encoding.chart_type_table:
            graph = DataGraph()
        else:
            raise ValueError("Cannot display chart of type {}".format(chart_type))

        return graph