Hello Klaus,
I have a problem with your xchart lib under b4j 7.31 even with an empty project in which only the xchart libary is involved comes the following error message.
In b4j 7 its works fine.
monki
B4J Version: 7.31
Java Version: 8
Parsing code. (0.10s)
Building folders structure. (0.03s)
Compiling code. (0.20s)
Compiling layout code. (0.00s)
Organizing libraries. (0.00s)
Compiling generated Java code. error
Can not find: D: \ PRG \ B4J 7.31 \ B4J \ libraries \ .jar