def area(<EOL>self,<EOL>x=None,<EOL>y=None,<EOL>stacked=True,<EOL>alpha=None,<EOL>var_name="<STR_LIT>",<EOL>value_name="<STR_LIT:value>",<EOL>width=<NUM_LIT>,<EOL>height=<NUM_LIT>,<EOL>ax=None,<EOL>**kwds<EOL>):
df = unpivot_frame(<EOL>self._data, x=x, y=y, var_name=var_name, value_name=value_name<EOL>)<EOL>x = df.columns[<NUM_LIT:0>]<EOL>if alpha is None and not stacked and df[var_name].nunique() > <NUM_LIT:1>:<EOL><INDENT>alpha = <NUM_LIT><EOL><DEDENT>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),<EOL>y=alt.Y(<EOL>value_name,<EOL>type=infer_vegalite_type(df[value_name]),<EOL>stack=(None, "<STR_LIT>")[stacked],<EOL>),<EOL>color=alt.Color(field=var_name, type=infer_vegalite_type(df[var_name])),<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 DataFrame data

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

        Parameters
        ----------
        x : string, optional
            the column to use as the x-axis variable. If not specified, the
            index will be used.
        y : string, optional
            the column to use as the y-axis variable. If not specified, all
            columns (except x if specified) will be used.
        stacked : bool, optional
            if True (default) then create a stacked area chart. Otherwise,
            areas will overlap
        alpha : float, optional
            transparency level, 0 <= alpha <= 1
        var_name : string, optional
            the legend title
        value_name : string, optional
            the y-axis label
        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