The change from plwid() to plwidth() changed the type of the argument from PLINT to PLFLT. However, this is not yet reflected in the documentation (e.g. http://plplot.sourceforge.net/docbook-manual/plplot-html-5.9.10/plwidth.html) which still states PLINT.
Thanks for spotting this issue. Fixed in svn trunk (which will be the source of our forthcoming 5.9.11 release).
Sorry for the "test", I can't delete my post! Please delete!