As in title , I try to develop as few schemes for hte 3d graphics enigne -- ogre3d ; problem is the big picture scheme the class Ogre::Root requires quite many arrows descending from it. The model of joining boxes with relational arrows is they can only go in / out by specific points around the border ( called "joints" or something ) . Now, however I wouldn't try there are always 4 + 4 + 3 + 3 joints displaced at four sides , suppose I need to place seven arrows on one side, how do I do that ?
EDIT: Here is what I mean, the "flies" or "crosses" to which one can bind the arrows are limited in number. I marked one of them with double circle ....
TL;DR Don't fight Dia to draw complex UML models. It is not appropriate tool for this task
1) the points around the border in Dia are called connection points. According to the official Dia FAQ their number is fixed in the shape's definition (see https://wiki.gnome.org/Apps/Dia/Faq → Why do elements have a fixed number of connection points?). However, it is possible to edit the shape definition and add your own points, see http://sdteffen.blogspot.cz/2012/08/improved-dia-shape-export-draw-your.html for more
2) using software designed for just drawing pictures for documenting UML models is not very smart neither useful. First and foremost such tools create pictures and can not work with or exchange UML models with other tools (see Wikipedia: XML Metadata Interchange for description of *.xmi format)
3) in the category of free editors for just pictures capable of drawing some simple UML diagrams I'd recommend the multiplatform yWorks yEd Graph Editor. An exceptional feature of this tool is the ability to automatically layout large graphs using several configurable algorithms and the ability to create nested/collapsible graphs + the output looks cool and the tool is quite easy to work with
4) in the category of real UML tools I'd recommend the Sparx Systems Enterprise Architect as commercial tool for Windows and Modeliosoft's Modelio as multiplatform open source tool. However, recommending a tool is out of the scope of the Stack Overflow site (§4) so I won't give you a comprehensive answer neither you'll find such a recommendation in older Stack Overflow answers also because the market of online html5 tools changes quickly and new tools appear quite often
