You can look at the spectrum of the observation line by clicking on that “Power Spectrum” button. The reason why the observation line functions differently than the observation point is because the latter directly records the time domain information of the field. To do this over a line (many points) or a observation area (many many points) is simply because it would take a lot of memory.
The power spectrum is a little more limited in that you need to set the parameters up before the simulation takes place. In Simulation Parameters you would need to modify the “DFT Options”.
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