This is correct, since the frequency is very different from point 1 to point 100.

You can extract the frequency, eg:

f=getdata("monitor","f");

then you can calculate wavelength. Please note that f(1) is the smallest corresponding to longest wavelength.

You may need to watch some online 100-level courses:

https://optics.ansys.com/hc/en-us/sections/360007813194-FDTD100-List-of-videos