Crosspost: Meaning of "fit ... error"?

[Philipp K. J. <ja...@ie...>]

I am cross-posting this from gnuplot-info.
The documentation is really not very clear.
Can somebody help clarify?

Best,

		Ph.


I am having difficulties understanding the
precise meaning of the "error" facility 
within the fit command. Can someone clarify?

1) The documentation mentions three "keywords":
	zerror
	yerror
	error z
Are they synonyms? Or are there semantic 
differences? Is "error z" a fixed phrase,
or is it also possible to have "error x"
or something like that?

2) In the demos I found an example using
"xyerror", which is not mentioned in the
docs. What does that do? How does weighting
the x-values enter into the calculation of
the error (=SSR)?

3) What does the presence of zerror actually
tell me (or gnuplot)? Is it correct to say 
that WITH "zerror" the last entry in the 
"using" spec is interpreted as the error 
column, whereas with "noerror", the last 
column is interpreted as the "y-value", and 
errors (=weights) are set equal to 1? Are
there exceptions to this rule?