As LsqFit.jl only calculates the standard error by using the Jacobian at the best-fit position I thought I try to write a small package for determining the confidence interval by data exploration.
https://github.com/feanor12/ConfidenceInterval.jl
As I am no expert I just implemented a method using an F-test and a simple linescan to test each parameter. (correlation is not taken into account yet, but creating a map in parameter space should not be that hard)
As this package is at a very early stage I wanted to check if something like this is already out there.