code: def line(self, x=None, y=None, **kwds):
          
          return self(kind='line', x=x, y=y, **kwds)
docs: Plot DataFrame columns as lines.
  
          This function is useful to plot lines using DataFrame's values
          as coordinates.
  
          Parameters
          ----------
          x : int or str, optional
              Columns to use for the horizontal axis.
              Either the location or the label of the columns to be used.
              By default, it will use the DataFrame indices.
          y : int, str, or list of them, optional
              The values to be plotted.
              Either the location or the label of the columns to be used.
              By default, it will use the remaining DataFrame numeric columns.
          **kwds
              Keyword arguments to pass on to :meth:`DataFrame.plot`.
  
          Returns
          -------
          :class:`matplotlib.axes.Axes` or :class:`numpy.ndarray`
              Return an ndarray when ``subplots=True``.
  
          See Also
          --------
          matplotlib.pyplot.plot : Plot y versus x as lines and/or markers.
  
          Examples
          --------
  
          .. plot::
              :context: close-figs
  
              The following example shows the populations for some animals
              over the years.
  
              >>> df = pd.DataFrame({
              ...    'pig': [20, 18, 489, 675, 1776],
              ...    'horse': [4, 25, 281, 600, 1900]
              ...    }, index=[1990, 1997, 2003, 2009, 2014])
              >>> lines = df.plot.line()
  
          .. plot::
             :context: close-figs
  
             An example with subplots, so an array of axes is returned.
  
             >>> axes = df.plot.line(subplots=True)
             >>> type(axes)
             <class 'numpy.ndarray'>
  
          .. plot::
              :context: close-figs
  
              The following example shows the relationship between both
              populations.
  
              >>> lines = df.plot.line(x='pig', y='horse')