Gnuplot fit line to two points?

It looks to me that the curve-fitting function is struggling to find the true parameters. This could be associated with the magnitude of your data points and/or trying to fit a line with two parameters to only two data points.

It looks to me that the curve-fitting function is struggling to find the true parameters. This could be associated with the magnitude of your data points and/or trying to fit a line with two parameters to only two data points. In any case, doing the calculation of a and be in Excel or equivalent yields: a= 577.769 be = -2233787 If you give gnuplot a good guess at what they should be, e.g. A=500 and b=-2233700 and repeat the procedure, it should successfully find the correct solution: Final set of parameters ======================= a = 577.769 be = -2.23379e+06 Of course, if you're fitting two points to a two-parameter straight line, it's much easier to calculate the values of a and be by hand: a = (9113-1602) / (3882-3869) be = 1602 - a * 3869.

Thanks for the answer. What do you mean saying: "magnitude of your data points"? – chriss Nov 21 '11 at 11:51 Gnuplot makes a first guess on the parameters (a=1.75, b=-1027) if one is unsupplied and iterates on these to get the true solution.In this case it looks like due to the relative sizes (i.e.

Magnitude) of your data points (1600-9000) the first guess that gnuplot makes is particularly bad, causing the fit function to fail. – William Menz Nov 21 '11 at 12:04 The thing is that for (3869. , 1.

) and (3870. , 2.) it also fails and the differences are much smaller. – chriss Nov 21 '11 at 13:21.

Gnuplot uses a non-linear method to determine the parameters of your function f with respect to a certain error value: limit for stopping : 1e-05. If you change that error value your function will be exactly fit. The error value can be specified with the FIT_LIMIT variable like so: FIT_LIMIT = 1e-8 With this setting your points will be exactly matched after 12 iterations.(At least on my machine^^).

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