Backwards compatibility can also be broken by changes in the ABI
(Application Binary Interface), not only by changes in the API. This can
happen, for instance, when there are changes in the structures that are made
public through plplot.h.
I documented this in the api-compat.xml chapter, whose updated HTML form is
Proofreading/comments are welcome.
I would like to create an automated test for ABI backwards incompatibility,
but I cannot imagine it right now (well, I did not think very much about
this yet). If you have any ideas, please tell me.