Hey Mike,
You could load a custom limit line which would in effect create a mask. Create a file in a text editor or in excel like so
904.0, -60
905.0, -60
905.0, -40
907.0, -40
907.0, -60
908.0, -60
(The format is [MHz, dBm] on each line)
and name it MyFileName.csv. Then in the software, File->Import->LimitLines and find and select your file.
If you create a list of points that outline the channels you are interested in observing, the software will create a max value limit line that will be drawn over the graticle. It would also let you know when a signal exceeds the limit line as well.
Is this type of functionality what you are looking for?
Regards,
A.J.