Hey, there. Currently, Eeschema <-> ngspice interface is somewhat limited so you can’t plot differential voltages directly in KiCad’s Simulator app. One workaround is to generate the difference voltage within the simulation itself and plot it. There are several ways to do this. Although E-source would probably be the most “correct” way, it requires a non-standard symbol with two extra control pins. Therefore, I opted to use a B-source in the attached example project (in ZIP file below) which uses the SPICE notation of V(node_a,node_b) to calculate the difference between node_a and node_b. You can also simply take the direct difference with V(node_a)-V(node_b). Both results are equivalent. Notice how I used global labels for my nodes since that removes the forward-slash required to reference them in the B-source equation.
dbridge_test.zip (4.5 KB)
Let me know if you have any follow-up questions.