[Elecraft] P3 fixed tune - no marker

[Dirk Jorens]

As well as a vertical line marker to indicate the tuned frequency in 
fixed tuning mode, I would like to see a better implementation of the 
passband indicator.  Being in the noise as it tends to be, it's 
continuously overwritten by the signal trace, making it hard to see.  
PowerSDR software programmers implemented the passband indicator as a 
different colored background. This might be a place to start. 73, Dirk
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