During my consequential research and exploration of UML and prototyping tools for Linux I found something I really enjoy way too much than expected. It's the Pencil Project.
Pencil is an open source tool based on Firefox for drawing UI prototypes, schemes, wireframes. It has a pretty interface which is usable and fast, it could be ran over Firefox or as a stand-alone application in most of the operating systems around. I had installed it on my Fedora station and now I enjoy drawing schemes for my clients since then.
The Pencil project offers some standard shapes for drawing, web components (h1 to h6, lorem ipsum, panels...), UI components from Forms (buttons, labels) and other popular things like tables that you might need to show the basic structure. Using a Windows Form/Frame as a parent component it looks exactly like a standard Swing or Forms application. Also, it has alignment of components by horizontal and vertical coordinates related to previously positioned elements. And so on. It's mandatory to try and use in the future.
Other more complex charts could be painted as well - it's up to you.