Ah, gotcha. Sadly, that’s not implemented currently, so for now you’d probably have to do it yourself. Way to go would probably be subclassing PPSD and overwriting the plot_spectrogram method..
from obspy.signal import PPSD
class MyPPSD(PPSD):
def plot_spectrogram(self, ...):
# copy in the code from obspy for this method and start tweaking it
Then use that MyPPSD as a drop in replacement for PPSD in your codes.
It shouldn’t be too hard to figure out what to change in the code, you can look at imaging/spectrogram.py for different means to plot those data and also look for traces of xaxis_frequency switch in signal/spectral_estimation.py for how to tweak things.