GUI: choose to display by monitor rather than spectrum number

Currently the DAE spectra plots ask for a spectrum number and use a SPEC:1:1:Y - it might be useful for a scientist to specify a monitor number, we should ask if they want a dropdown by each plot asking for "spectrum" or "monitor". From a GUI point of view you just need to ask for e.g. MON:1:1:Y if they request monitor 1, would avoid needing to look up monitor -> spectrum conversion that may change with wiring tables