From the site: UMLet is an open-source UML tool with a simple user interface: draw UML diagrams fast, export diagrams to eps, pdf, jpg, svg, and clipboard, share diagrams using Eclipse, and create new, custom UML elements. In my opinion it's a very good and useful tool. The only alternatives I managed to find in portage are bouml and argouml. Both are good but I think UMLet produces more professional UML diagrams. Reproducible: Always
I also vote for including this tool in portage. Its clean interface and usability are charming indeed.
I also vote for including Umlet, as I too consider it very useful. The Czech Technical University in Prague has Umlet in their overlay. https://ict.fit.cvut.cz/gitlab/gentoo/overlay/tree/master/dev-util/umlet