Hi Alexei,
The schema of jPDL is included by the jBPM distribution file. You could get it from the http://sourceforge.net/projects/jbpm/files/。
As you can see in the .jpdl.xml, the latest version is 4.3, so you can find out there is a file named jpdl-4.3.xsd in the src directory.
The distribution file includes a Graphical Process Designer, it is a eclipse plugin, you could use it to design jPDL process definitions. You could find more information in the User Guide. http://docs.jboss.com/jbpm/v4/userguide/html_single/#graphicalprocessdesigner
In my opinion, the season why jPDL didn't seperate the g="1,2,3,4" to x="1" y="1" w="1" h="1" is the transition could have more than one point in the g attribute, but I don't very clearly for that, maybe someone else could give us the reason of this.