Ngspice postprocessing
Some hints:

There are several third partyinterfaces  that can help you manipulate waveforms.
I saw that EasyEDA claims:


  Once saved in a Project, a WaveForm window can be exported as a .pdf, .png or .svg file into your browser window. This can then be saved to your device so it is easy to create professional quality documentation.


Searching internet on "ngspice waveform viewer" I found: post-simulation data processing with Qucs-S and Octave