Allowing the use of the dB and dBm scales when the user calculates signals using equation

[csrabak]

Presently the Cartesian diagram has an selectable menu for "units" which include dB and dBm, but they're not selectable if the signal isn't generated by Qucs-S itself, for example if the signal vector is from an equation using the dB function (say in NGspice).

I think it would be worthwhile to allow this on user's discretion.