It may not be exactly what you want, but I tend to use OpenOffice (or LibreOffice) Draw. It supports numerous shapes and more importantly connectors between shapes, as well as adding custom connection points to shapes.
Not as good as Pre-MS acquisition Visuo was, but I prefer Draw to MS Visio (though haven't used it in a while).
See:
https://www.libreoffice.org
or
http://www.openoffice.org
