Draw a line between electrodes listed in a route.

        Arguments
        ---------

         - `df_route`:
             * A `pandas.DataFrame` containing a column named `electrode_i`.
             * For each row, `electrode_i` corresponds to the integer index of
               the corresponding electrode.
         - `cr`: Cairo context.
         - `color`: Either a RGB or RGBA tuple, with each color channel in the
           range [0, 1].  If `color` is `None`, the electrode color is set to
           white.

def draw_route(self, df_route, cr, color=None, line_width=None):
        '''
        Draw a line between electrodes listed in a route.

        Arguments
        ---------

         - `df_route`:
             * A `pandas.DataFrame` containing a column named `electrode_i`.
             * For each row, `electrode_i` corresponds to the integer index of
               the corresponding electrode.
         - `cr`: Cairo context.
         - `color`: Either a RGB or RGBA tuple, with each color channel in the
           range [0, 1].  If `color` is `None`, the electrode color is set to
           white.
        '''
        df_route_centers = (self.canvas.df_shape_centers
                            .ix[df_route.electrode_i][['x_center',
                                                       'y_center']])
        df_endpoint_marker = (.6 * self.get_endpoint_marker(df_route_centers)
                              + df_route_centers.iloc[-1].values)

        # Save cairo context to restore after drawing route.
        cr.save()
        if color is None:
            # Colors from ["Show me the numbers"][1].
            #
            # [1]: http://blog.axc.net/its-the-colors-you-have/
            # LiteOrange = rgb(251,178,88);
            # MedOrange = rgb(250,164,58);
            # LiteGreen = rgb(144,205,151);
            # MedGreen = rgb(96,189,104);
            color_rgb_255 = np.array([96,189,104, .8 * 255])
            color = (color_rgb_255 / 255.).tolist()
        if len(color) < 4:
            color += [1.] * (4 - len(color))
        cr.set_source_rgba(*color)
        cr.move_to(*df_route_centers.iloc[0])
        for electrode_i, center_i in df_route_centers.iloc[1:].iterrows():
            cr.line_to(*center_i)
        if line_width is None:
            line_width = np.sqrt((df_endpoint_marker.max().values -
                                  df_endpoint_marker.min().values).prod()) * .1
        cr.set_line_width(4)
        cr.stroke()

        cr.move_to(*df_endpoint_marker.iloc[0])
        for electrode_i, center_i in df_endpoint_marker.iloc[1:].iterrows():
            cr.line_to(*center_i)
        cr.close_path()
        cr.set_source_rgba(*color)
        cr.fill()
        # Restore cairo context after drawing route.
        cr.restore()