Is there a possibility to generate in eye-diagram with xschem and ngspice?

I think you first need to decide how you model clock jitter, supply noise and any other kind of disturb. Then you need to run multiple simulations and save multiple datasets. Both ngspice (for the simulation and wave display) and xschem (for schematic and wave display) support multiple simulation runs.

Thank you for your quick response! Initially I was thinking of just running one long simulation and merging many periods into one big picture. But without any modeling of clock jitter etc. this might not give a reasonable result....