def plot_residuals(self, plot=None):
        """ Plot normalized fit residuals.

        The sum of the squares of the residuals equals ``self.chi2``.
        Individual residuals should be distributed about one, in
        a Gaussian distribution.

        Args:
            plot: :mod:`matplotlib` plotter. If ``None``, uses
                ``matplotlib.pyplot`.

        Returns:
            Plotter ``plot``.
        """
        if plot is None:
            import matplotlib.pyplot as plot
        x = numpy.arange(1, len(self.residuals) + 1)
        y = _gvar.mean(self.residuals)
        yerr = _gvar.sdev(self.residuals)
        plot.errorbar(x=x, y=y, yerr=yerr, fmt='o', color='b')
        plot.ylabel('normalized residuals')
        xr = [x[0], x[-1]]
        plot.plot([x[0], x[-1]], [0, 0], 'r-')
        plot.fill_between(
            x=xr, y1=[-1,-1], y2=[1,1], color='r', alpha=0.075
            )
        return plot
Plot normalized fit residuals.

        The sum of the squares of the residuals equals ``self.chi2``.
        Individual residuals should be distributed about one, in
        a Gaussian distribution.

        Args:
            plot: :mod:`matplotlib` plotter. If ``None``, uses
                ``matplotlib.pyplot`.

        Returns:
            Plotter ``plot``.