Get an Altair chart object

def _get_altair_chart(self, xfield, yfield, chart_type,
                          label, opts={}, style={}, **kwargs):
        """
        Get an Altair chart object
        """
        encode = self.altair_encode
        chart = None
        if chart_type == "bar":
            chart = Chart(self.df).mark_bar(**style).encode(x=xfield, \
                                            y=yfield, **encode).properties(**opts)
        elif chart_type == "circle":
            chart = Chart(self.df).mark_circle(**style).encode(x=xfield, \
                                            y=yfield, **encode).properties(**opts)
        elif chart_type == "line":
            chart = Chart(self.df).mark_line(**style).encode(x=xfield, \
                                            y=yfield, **encode).properties(**opts)
        elif chart_type == "hline":
            chart = self._altair_hline_(xfield, yfield, opts, style, encode)
        elif chart_type == "line_num":
            chart = self._altair_line_num_(xfield, yfield, opts, style, encode)
        elif chart_type == "bar_num":
            chart = self._altair_bar_num_(xfield, yfield, opts, style, encode)
        elif chart_type == "point_num":
            chart = self._altair_point_num_(xfield, yfield, opts, style, encode)
        elif chart_type == "point":
            chart = Chart(self.df).mark_point(**style).encode(x=xfield, \
                                            y=yfield, **encode).properties(**opts)
        elif chart_type == "area":
            chart = Chart(self.df).mark_area(**style).encode(x=xfield, \
                                            y=yfield, **encode).properties(**opts)
        elif chart_type == "heatmap":
            chart = Chart(self.df).mark_rect(**style).encode(x=xfield, \
                                            y=yfield, **encode).properties(**opts)
        elif chart_type == "text":
            chart = Chart(self.df).mark_text(**style).encode(x=xfield, \
                                            y=yfield, **encode).properties(**opts)
        elif chart_type == "square":
            chart = Chart(self.df).mark_square(**style).encode(x=xfield, \
                                                y=yfield, **encode).properties(**opts)
        elif chart_type == "tick":
            chart = Chart(self.df).mark_tick(**style).encode(x=xfield, \
                                            y=yfield, **encode).properties(**opts)
        elif chart_type == "rule":
            chart = Chart(self.df).mark_rule(**style).encode(x=xfield, \
                                            y=yfield, **encode).properties(**opts)
        return chart