Hi,
I'm seeing in 'manage custom properties' a way to add a list of custom properties to either edges/nodes.
The problem i see here is 2 fold.
1) the list of properties is defined for all edges/nodes.
  so if i want to defined a 6th one, but in the custom properties i provisionned only 5 then i have to add another one.... i guess it's possible to provition for that extra number without problem
2) custom properties have a common key name. 
So if we need to use it even with other 2 limitations, it forces us to basically name custome porperties like property_n. where n is in [1-*]
3)  more important issue, though is that the properties are not displayable at all. Not even togglizable. It'd be great to have the flexibility of the edges/nodes label positioninig for each and every properties.
I'm still searching for a tool that would allow to create graphs like that shown here 
http://pelle.stephane.free.fr/uml_vs_hbds.pdf
but Really, there seems to be none. 
yEd other than this apparent missing feature is a great piece of engineering.
I hope you can take this positively to further enhance yEd
 
keep up the good work guys