sine-waves | Generate multiple configurable sine waves | Canvas library

There are a few issues here that I do not understand:

1. There is no need to define the fit function inside the "fit function"
2. There is no need to define it twice if the only difference is the naming of the dictionary. (While I do not understand why this has to be named differently in the first place)
3. One could directly fit the frequency instead of omega
4. When pre-calculating the fitted values, directly use the given fitfunction

Overall I don't see why the second fit should fail and using some generic data here, it doesn't. Considering the fact that in physics an amplitude can be complex I don't have a problem with a negative results. Nevertheless, I understand the point in the OP. Surely, a fit algorithm does not know about physics and, mathematically, there is no problem with the amplitude being negative. This just gives an additional phase shift of pi. Hence, one can easily force positive amplitudes when taking care of the required phase shift. I introduced this here as possible keyword argument. Moreover I reduced this to one fit function with possible "renaming" of the output dictionary keys as keyword argument.