def line(<EOL>self,<EOL>x=None,<EOL>y=None,<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>):
use_order = (x is not None)<EOL>if use_order:<EOL><INDENT>df = self._data.reset_index()<EOL>order = df.columns[<NUM_LIT:0>]<EOL>df = unpivot_frame(<EOL>df, x=(x, order), y=y, var_name=var_name, value_name=value_name<EOL>)<EOL><DEDENT>else:<EOL><INDENT>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><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_line().encode(<EOL>x=_x(x, df), y=_y(value_name, df), color=alt.Color(var_name, type="<STR_LIT>")<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 use_order:<EOL><INDENT>chart.encoding["<STR_LIT>"] = {<EOL>"<STR_LIT>": order, "<STR_LIT:type>": infer_vegalite_type(df[order])<EOL>}<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>
Line plot for DataFrame data

        >>> dataframe.vgplot.line()  # 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.
        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