As suggested run the electronics off a battery that is not used for engine start. This may require a wiring change if you have a 1,2,both switch and 2 batteries.
If you have only one battery or rewire is difficult. You could try fitting a silicon diode in series with the +ve supply to the plotter and also one for the GPS. Or move the supply to both so they both go through the diode. You then fit a large electrolytic capacitor rated at more than 15 volts and preferably in the 10000 microfarad range. Bigger is better. Or add several capacitors in paralell.

The capacitor is wired +ve to -ve on the side of the diode towards the GPS/ plotter. The capcitor charges a bit like a small capacity battery to the battery voltage but when the engine is started and the voltage falls the capacitor will provide power to the GPS/ plotter. The diode stops the capacitor voltage from running back to the engine starter.
A small SLA battery would do the same job but would need switching to disconnect when you power off to avoid discharge. Tyhe voltage drop of the diode hopefuilly will be enough to avoid over chage of the battery. But really the capacitor is much less concern. It is happy to be completely discharged on power off and wiull last forever. (almost) But it might need a larger capacitor if your engine is cranked for a long time. The time the capacitor will support your GPS /plotter depends on how much current your plotter and GPS draw.

Capacitors are fairly cheap from Maplin and can be paralelled to increase capacity. Diode should be rated for several amps and are also cheap from Maplin RS etc. good luck olewill