Since PLplot is developed by so many people, the task of checking for backwards compatibility of the library is very hard. As for the 5.3.1 release, we do not have any rigorous regression test for check whether the library is really backwards compatible.
However, here are some rules to be followed by the Release Manager prior to releasing a new version of PLplot:
	    Check if there are any changes in
	    plplot.h.  If no prototype is changed,
	    then the chances are high that no backwards incompatibilities
	    have been introduced.  If new functions has been added, then
	    the library soname will be kept, although the soversion
	    strings in cmake/module/plplot_version.cmake
	    must be changed following the instructions in that file.
	  
	    A necessary, but not sufficient test consists of the following:
	    first, install the previous released version of PLplot in the
	    system and compile all the examples
	    examples/c/x??c..  After that, install the
	    to-be-released version of PLplot and try to run the previously
	    compiled examples.  If they either link or run incorrectly,
	    then backwards incompatibilities have been introduced and the
	    soversion string must be upgraded from x:y:z
	    to (x+1):0:0.