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
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