The eeschema-ngspice interface is still rather limited. You cannot simply plot the voltage difference between two nodes.
So you have to create (design) your own voltmeter that provides a differential to single ended conversion.
Place a three terminal symbol with nodes 1, 2, 3. Assign 1, 2, 3 to in+, in- and out nodes. place a resistor from out to ground (to not have an unconnected output).
Assign an ngspice subcircuit to your new device that uses a ngspice voltage controlled voltage source vcvs inside, which produces a voltage difference between its in+ and in- nodes and amplifies the difference by factor 1 and sends this to node out.
.subckt meas in+ in- out
Emeas out 0 in+ in- 1
Now measure (plot) the ac voltage at node ‘out’. This will be the ratio to V1 if you assign ‘DC 0 AC 1’ to V1, not just AC, and you will get the bode plot (gain and phase versus frequency), if you run an ac simulation.