Looks like you miswired in your 'Build XY Graph2' Express VI - you wired a constant which was the XScale.Multipler of the waveform graph You then write this constant to your measurement file, which is maybe why you are only getting the highest value?
I think want you want to wire is the 'Result' in the Formula Express VI above it, which presumably takes tne array of pixel values (0...511), and gives you the wavelength.for each pixel. Then use this as the x values in your Build XY Graph