weights required for 2D fit

[Petr M. <mi...@ph...>]

A colleague has noticed wrong parameter values after 2D fitting via
     fit f(x,y) 'data.txt' us 1:2:3 via a,b,c

Gnuplot was fitting correctly after adding the dummy weight column as
"help fit" indicates:
     fit f(x,y) 'data.txt' us 1:2:3:(1) via a,b,c

I think gnuplot should write an error of not having enough columns or 
missing weights column in the former command or it should fill weights by 
one if the 4th column is missing.

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PM