> What is the difference between "using 1:2:3 and using 1:2:3:(1)" for 2d fit
Primarily that one of them is _not_ a 2D fit. Which of those that is
depends on what exactly you mean by "2D" fit.
> fit f(x,y) "nor.dat" using 1:2:3 via a,b,c
This won't work. A fit with 2 independent variables needs four using
specifications: x, y, z and delta_z. If your data file lacks a delta_z
column, you have to invent one, like that:
> fit f(x,y) "nor.dat" using 1:2:3:(1) via a,b,c
And since all of this is described right on the first page of "help
fit", I really wonder how you managed to miss this.