'Initialize the *line* artist that is responsible to draw the axis line.'

def _init_line(self):

    tran = (self._axis_artist_helper.get_line_transform(self.axes) + self.offset_transform)
    axisline_style = self.get_axisline_style()
    if (axisline_style is None):
        self.line = BezierPath(self._axis_artist_helper.get_line(self.axes), color=rcParams[u'axes.edgecolor'], linewidth=rcParams[u'axes.linewidth'], transform=tran)
    else:
        self.line = axisline_style(self, transform=tran)
