Yes, well, I'm afraid I don't have a 50" screen in order to actually figure out your VI. I couldn't make heads or tails out of it due to the gorge of local variables.
As far as I could tell, it seems that you're trying to access the "Current wavelength" that's being updated in another loop. The way you have it you're reading the value before you start the loop that's updating the graph. This means that even if this value is changed, the loop only ever sees the original value. If you place this local variable inside that loop you should get the value as it's being updated.
I can say definitevily that the one case structure that you have that's driven by a local variable of "Stepsize" unnecessarily duplicates code. You're essentially doing the same thing in each case. I only saw one minor difference. Rather than having a case around the whole code, have the case around this just one bit of difference. Easier maintenance.
You're also using both VISA and old-style GPIB functions. Stick with VISA. The old-style GPIB functions should not be used for new code. They are there for compatibility with old (like more than 10 years ago) code.
Please, please, clean up the code, and try to pare down all those local variables.