I want to thank you for your replies and your suggestions.
A collegue from the University of Trieste replied directly to me with a python script with the following work flow which is similar to the suggestions I received via the Obspy list:
1. put all mseed files which should be plot together into one folder
2. iterate through all files and within this folder, read into streams and cut all to the desired length, starting with the p pick time and write cut streams to a second directory
3. now iterate through the cut streams:
Read trace of interest (e.g. "HHZ")
fill an np array with the data, starting at zero time, amplitudes normalized
add subplot to plot array of matplotlib.pyplot
4. finally show plot/subplots on screen or write to graphics output file
The script basically runs but problems still exist due to minimal time shifts (0.0003 seconds) which can appear in the start time of some traces. This causes matplotlib.pyplot to rise an error because of the small shift in the x axis. I will be trying to find out if there is a way to force the trim function to avoid these shifts.