Fix plot_xyfit

residuals plotted with plot_xyfit where wrong due to miscalculated
squares of the residuals.

Minor fix of xyfit_fun now setups the function to return an array of length(x)
if only one parameter is given as in > xyfit_fun("constant");
Previously only the parameter value was returned. This does not affect
the fitting but leads to missbehaving plotting routines.