def area(self, alpha=None, width=<NUM_LIT>, height=<NUM_LIT>, ax=None, **kwds):
df = self._data.reset_index()<EOL>df.columns = map(str, df.columns)<EOL>x, y = df.columns<EOL>chart = self._plot(<EOL>data=df,<EOL>width=width,<EOL>height=height,<EOL>title=kwds.pop("<STR_LIT:title>", "<STR_LIT>"),<EOL>figsize=kwds.pop("<STR_LIT>", None),<EOL>dpi=kwds.pop("<STR_LIT>", None),<EOL>).mark_area().encode(<EOL>x=_x(x, df), y=_y(y, df)<EOL>)<EOL>if alpha is not None:<EOL><INDENT>assert <NUM_LIT:0> <= alpha <= <NUM_LIT:1><EOL>chart = chart.encode(opacity=alt.value(alpha))<EOL><DEDENT>if ax is not None:<EOL><INDENT>return ax + chart<EOL><DEDENT>warn_if_keywords_unused("<STR_LIT>", kwds)<EOL>return chart<EOL>
Area plot for Series data

        >>> series.vgplot.area()  # doctest: +SKIP

        Parameters
        ----------
        alpha : float, optional
            transparency level, 0 <= alpha <= 1
        width : int, optional
            the width of the plot in pixels
        height : int, optional
            the height of the plot in pixels
        ax: altair.Chart, optional
            chart to be overlayed with this vis (convinience method for `chart1 + chart2`)

        Returns
        -------
        chart : alt.Chart
            altair chart representation