If you know the period and want to fit phase and amplitude, this is
equivalent to fitting a * sin + b * cos
> >>> > I don't know how to set the approximate starting values.
I'm not sure what you meant by that, but I suspect it's related to
phase and amplitude.
> >>> > Besides, does the method work for sine curve as well?
sin is the same as cos with a different phase
Any combination of a and b above = c * sin (theta + d) for
some value of c and d and = e * cos (theta + f) for some value
of e and f.
Also for any c,d and for any e,f there is an a,b.
the c and e are what I'm calling amplitude, the d and f are what
I'm calling phase.