Plots look wrong or they are not shown at all in octave. Problem is related to graphics toolkits 'qt' (the default one) and 'fltk'. It happens when using octave from PureOS amber repositories (version 4.4.1) but also with the latest octave version (through flatpak) and the development version obtained from Octave repository. It does not happen in Debian 10 (tested in a virtual machine).
Example to reproduce:
octave:1> x=0:0.1:10; y=sin(x);
octave:2> plot(x,y, '+r')
I include a screenshot of an example of correct behavior in Debian 10 and what is shown in PureOS amber.
As a workaround, 'gnuplot' toolkit can be used
and plots seem to be displayed (more) correctly.
I am available to help if it is needed.