Can you please provide me code where you have implemented part with signal represented in frequency domain and also proper settings for cursor which you would like to use? Please, make code as simple as posible.
To implement logic which you would like, I suggest you to use Property nodes, which enables you to read and set values of cursor and waveform graph. To use those properties right clck on graph, select option Create->Property node-> X scale->Range and choose paramiters which can be usefull for solving your issue.
Also, if you can use paramiters of your cursor if you use property nodes (Righ click graph on front panel, select option Create->Property node->Cursor).
Try to implement your logic using property nodes which I mention. If you do not manage to do this, please send me code which I asked in begining in my post, and I will assist you with implementation o your logic.